UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 6-K

REPORT OF FOREIGN PRIVATE ISSUER

PURSUANT TO RULE 13a-16 OR 15d-16 UNDER

THE SECURITIES EXCHANGE ACT OF 1934

For the month of March 2019

Commission File Number: 001-35126

21Vianet Group, Inc.

M5, 1 Jiuxianqiao East Road,

Chaoyang District

Beijing 100016

The People’s Republic of China

(86 10) 8456 2121

(Address, including zip code, and telephone number, including area code, of Registrant’s principal executive offices)

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F.

Form 20-F  ☒            Form 40-F  ☐

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1):  ☐

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7):  ☐

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

Date: March 6, 2019

Exhibit Index

Exhibit 99.1 — Press Release

Exhibit 99.1

21Vianet Group, Inc. Reports Unaudited Fourth Quarter and Full Year 2018 Financial Results

BEIJING, March 5, 2019 (GLOBE NEWSWIRE) — 21Vianet Group, Inc. (Nasdaq: VNET) (“21Vianet” or the “Company”), a leading carrier- and cloud-neutral Internet data center services provider in China, today announced its unaudited financial results for the fourth quarter and the full year ended December 31, 2018. The Company will hold a conference call at 8:00 pm on Monday, March 4, 2019, U.S. Eastern Time to discuss the financial results. Dial-in details are provided at the end of this release.

Fourth Quarter 2018 Financial Highlights

Full Year 2018 Financial Highlights (including hosting and related business only)

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Fourth Quarter 2018 Operational Highlights

“We concluded a fruitful 2018 with a solid fourth quarter performance,” commented Mr. Alvin Wang, Chief Executive Officer and President of the Company. “Our resilient financial growth and improving operating performance once again demonstrated the effectiveness of our business operation optimization and our ability to capitalize on the increasing market demand for high-quality data hosting, hybrid IT, and cloud services. To sustain our strong growth momentum, we actively acquired new land resources and new customers while expanding our footprint to new Tier-1 markets and beyond. In addition, we continued to deepen our strategic partnerships with world-class technology companies such as Red Hat Inc. to explore more solutions that can empower our customers’ business expansions. Looking ahead to 2019 and beyond, we are confident that the counter-cyclical nature of our business as well as our leadership in the Chinese IDC market will facilitate our pursuit of long-term sustainable growth.”

Ms. Sharon Liu, Chief Financial Officer of the Company, commented, “We delivered a strong quarter with healthy top- and bottom-line growth. In the fourth quarter of 2018, we had net revenues of RMB901.9 million, again exceeding the high end of our previous guidance range. More importantly, our adjusted EBITDA margin further increased to 28.3%, maintaining its consistent improvement over the previous quarters. In 2018, we improved adjusted EBITDA margin significantly to 27.0% from 22.5% in the previous year. In 2019, we are confident that as we continue to execute on our growth strategies and fortify our market leading position, we will yield long-term value for our shareholders.”

Fourth Quarter 2018 Financial Results

REVENUES: Net revenues increased by 17.8% to RMB901.9 million (US$131.2 million) in the fourth quarter of 2018 from RMB765.8 million in the same period of 2017 and increased by 3.7% from RMB870.1 million in the third quarter of 2018. The increase was primarily attributable to the growing demand for data centers and cloud services in the domestic market.

GROSS PROFIT: Gross profit increased by 23.1% to RMB246.3 million (US$35.8 million) in the fourth quarter of 2018 from RMB200.2 million in the same period of 2017 and increased by 2.1% from RMB241.2 million in the third quarter of 2018. Gross margin was 27.3% in the fourth quarter of 2018 compared to 26.1% in the same period of 2017 and 27.7% in the third quarter of 2018. The year-over-year improvement in gross margin was mainly attributable to the Company’s continuous efforts in maximizing its operating efficiency.

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ADJUSTED CASH GROSS PROFIT, which excludes depreciation, amortization, and share-based compensation expenses, increased by 27.9% to RMB409.2 million (US$59.5 million) in the fourth quarter of 2018 from RMB320.1 million in the same period of 2017 and increased by 4.4% from RMB391.9 million in the third quarter of 2018. Adjusted cash gross margin expanded to 45.4% in the fourth quarter of 2018 from 41.8% in the same period of 2017 and 45.0% in the third quarter of 2018.

OPERATING EXPENSES: Total operating expenses decreased by 5.7% to RMB181.4 million (US$26.4 million) in the fourth quarter of 2018 from RMB192.4 million in the same period of 2017 and increased by 2.8% from RMB176.6 million in the third quarter of 2018. As a percentage of net revenues, total operating expenses decreased to 20.1% in the fourth quarter of 2018 from 25.1% in the same period of 2017 and 20.3% in the third quarter of 2018. The decrease of operating expenses as a percentage of net revenues was primarily due to the successful implementation of the Company’s efficiency enhancement initiatives.

Sales and marketing expenses were RMB49.2 million (US$7.2 million) in the fourth quarter of 2018 compared to RMB42.7 million in the same period of 2017 and RMB39.9 million in the third quarter of 2018. The increase was mainly attributable to increased marketing activities and higher sales commissions, which was in line with the growth of the Company’s net revenues in the fourth quarter of 2018.

Research and development expenses were RMB23.6 million (US$3.4 million) in the fourth quarter of 2018 compared to RMB29.3 million in the same period of 2017 and RMB24.3 million in the third quarter of 2018.

General and administrative expenses were RMB131.0 million (US$19.0 million) in the fourth quarter of 2018 compared to RMB115.4 million in the same period of 2017 and RMB110.2 million in the third quarter of 2018. The increase was mainly attributable to share-based compensation expenses that the Company recognized during the fourth quarter of 2018.

ADJUSTED OPERATING EXPENSES, which exclude share-based compensation expenses and changes in the fair value of contingent purchase consideration payables, decreased by 0.5% to RMB172.4 million (US$25.1 million) in the fourth quarter of 2018 from RMB173.2 million in the same period of 2017 and increased by 5.9% from RMB162.9 million in the third quarter of 2018. As a percentage of net revenues, adjusted operating expenses decreased to 19.1% in the fourth quarter of 2018 from 22.6% in the same period of 2017 and increased slightly from 18.7% in the third quarter of 2018.

ADJUSTED EBITDA: Adjusted EBITDA in the fourth quarter of 2018 increased by 49.3% to RMB255.3 million (US$37.1 million) from RMB171.0 million in the same period of 2017 and increased by 4.1% from RMB245.2 million in the third quarter of 2018. Adjusted EBITDA in the fourth quarter of 2018 excluded share-based compensation expenses of RMB29.2 million (US$4.2 million) and changes in the fair value of contingent purchase consideration payables, which was a gain of RMB18.5 million (US$2.7 million). Adjusted EBITDA margin expanded to 28.3% in the fourth quarter of 2018 from 22.3% in the same period of 2017 and 28.2% in the third quarter of 2018.

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NET PROFIT/LOSS ATTRIBUTABLE TO ORDINARY SHAREHOLDERS: Net loss attributable to ordinary shareholders in the fourth quarter of 2018 was RMB114.1 million (US$16.6 million) compared to a net profit of RMB798.6 million in the same period of 2017 and a net loss of RMB29.6 million in the third quarter of 2018. Net loss attributable to ordinary shareholders in the fourth quarter of 2018 included a foreign exchange gain of RMB2.5 million (US$0.4 million) compared to RMB4.3 million in the same period of 2017 and a loss of RMB55.0 million in the third quarter of 2018.

PROFIT/LOSS PER SHARE: Basic and diluted loss per share were RMB0.17 (US$0.02) in the fourth quarter of 2018, which represents the equivalent of RMB1.02 (US$0.12) per American Depositary Share (“ADS”). Each ADS represents six ordinary shares. Diluted earnings per share is calculated using net earnings attributable to ordinary shareholders divided by the weighted average number of diluted shares outstanding.

As of December 31, 2018, the Company’s cash and cash equivalents, restricted cash, and short-term investments were RMB2.91 billion (US$422.7 million).

Net cash generated from operating activities was RMB237.0 million (US$34.5 million) in the fourth quarter of 2018 compared to RMB157.1 million in the same period of 2017 and RMB260.7 million in the third quarter of 2018.

Full Year 2018 Financial Results

To fully reflect the Company’s performance, all analysis between “REVENUES” and “ADJUSTED EBITDA” present only the results of the hosting and related service business. The MNS business, which was disposed of in the third quarter of 2017, is excluded.

REVENUES: Net revenues increased by 14.3% to RMB3.40 billion (US$494.7 million) in 2018 from RMB2.98 billion in the prior year. The increase was primarily due to the same factors that led to the quarterly increase.

GROSS PROFIT: Gross profit increased by 11.8% to RMB944.9 million (US$137.4 million) in 2018 from RMB844.9 million in the prior year. Gross margin was 27.8% in 2018 compared to 28.4% in the prior year.

ADJUSTED CASH GROSS PROFIT, which excludes depreciation, amortization, and share-based compensation expenses, increased by 21.2% to RMB1.51 billion (US$220.0 million) in 2018 from RMB1.25 billion in the prior year. Adjusted cash gross margin expanded to 44.5% in 2018 from 42.0% in the prior year.

OPERATING EXPENSES: Total operating expenses increased by 2.8% to RMB707.4 million (US$102.9 million) in 2018 from RMB688.3 million in the prior year. As a percentage of net revenues, total operating expenses decreased to 20.8% in 2018 from 23.1% in the prior year. The decrease of operating expenses as a percentage of net revenues was primarily due to the successful implementation of the Company’s efficiency enhancement initiatives.

Sales and marketing expenses were RMB172.2 million (US$25.0 million) in 2018 compared to RMB171.8 million in the prior year.

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Research and development expenses were RMB92.1 million (US$13.4 million) in 2018 compared to RMB97.6 million in the prior year.

General and administrative expenses were RMB462.6 million (US$67.3 million) in 2018 compared to RMB417.2million in the prior year.

ADJUSTED OPERATING EXPENSES, which exclude share-based compensation expenses and changes in the fair value of contingent purchase consideration payables, increased by 3.8% to RMB664.4 million (US$96.6 million) in 2018 from RMB639.9 million in the prior year. As a percentage of net revenues, adjusted operating expenses decreased to 19.5% in 2018 from 21.5% in the prior year.

ADJUSTED EBITDA: Adjusted EBITDA in 2018 increased by 36.8% to RMB917.7 million (US$133.5 million) from RMB670.8 million in the prior year. Adjusted EBITDA in 2018 excluded share-based compensation expenses of RMB59.5 million (US$8.7 million) and changes in the fair value of contingent purchase consideration payables, which was a gain of RMB13.9 million (US$2.0 million). Adjusted EBITDA margin expanded to 27.0% in 2018 from 22.5% in the prior year.

NET PROFIT/LOSS ATTRIBUTABLE TO ORDINARY SHAREHOLDERS: Net loss attributable to ordinary shareholders for 2018 was RMB205.1 million (US$29.8 million) compared to a net loss of RMB772.7 million in the prior year. Net loss in 2018 included a foreign exchange loss of RMB81.1 million (US$11.8 million) compared to RMB17.2 million in the prior year.

PROFIT/LOSS PER SHARE: Basic and diluted loss per share were RMB0.30 (US$0.04) for 2018, which represents the equivalent of RMB1.80 (US$0.24) per ADS. Diluted loss per share is calculated using net loss attributable to ordinary shareholders divided by the weighted average number of diluted shares outstanding.

Net cash generated from operating activities was RMB705.0 million (US$102.5 million) in 2018 compared to RMB487.2 million in 2017.

Recent Development

In February 2019, the Company reached an agreement to acquire a recently-constructed data center in urban Chengdu City, Sichuan Province, China. Upon the completion of quality inspections, the new data center is expected to add approximately 500 cabinets to the Company’s sales pipeline. This additional capacity will enable 21Vianet to access the new Tier-1 market in Chengdu, the primary economic center and core network hub for Southwest China, where the customer demand for high-quality data center services has been growing.

Financial Outlook

For the first quarter of 2019, the Company expects net revenues to be in the range of RMB860 million to RMB880 million. Adjusted EBITDA is expected to be in the range of RMB230 million to RMB250 million.

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For the full year of 2019, the Company expects net revenues to be in the range of RMB3,760 million to RMB3,860 million. Adjusted EBITDA is expected to be in the range of RMB1,000 million to RMB1,100 million. The midpoints of the Company’s updated estimates imply an increase of 12% year-over-year in total revenues and an increase of 14% year-over-year in adjusted EBITDA.

The forecast reflects the Company’s current and preliminary view on the market and its operational conditions, which is subject to change.

Conference Call

The Company will hold a conference call at 8:00 pm on Monday, March 4, 2019 U.S. Eastern Time, or 9:00 am on Tuesday, March 5, 2019 Beijing Time, to discuss the financial results.

Participants may access the call by dialing the following numbers:

The replay will be accessible through March 12, 2019 by dialing the following numbers:

A live and archived webcast of the conference call will be available through the Company’s investor relation website at http://ir.21vianet.com.

Non-GAAP Disclosure

In evaluating its business, 21Vianet considers and uses the following non-GAAP measures defined as non-GAAP financial measures by the SEC as supplemental measure to review and assess its operating performance: adjusted cash gross profit, adjusted cash gross margin, adjusted operating expenses, adjusted EBITDA, adjusted EBITDA margin, The presentation of these non-GAAP financial measures is not intended to be considered in isolation or as a substitute for the financial information prepared and presented in accordance with U.S. GAAP. For more information on these non-GAAP financial measures, please see the table captioned “Reconciliations of GAAP and non-GAAP results” set forth at the end of this press release.

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The non-GAAP financial measures are provided as additional information to help investors compare business trends among different reporting periods on a consistent basis and to enhance investors’ overall understanding of the Company’s current financial performance and prospects for the future. These non-GAAP financial measures should be considered in addition to results prepared in accordance with U.S. GAAP, but should not be considered a substitute for, or superior to, U.S. GAAP results. In addition, the Company’s calculation of the non-GAAP financial measures may be different from the calculation used by other companies, and therefore comparability may be limited.

Exchange Rate

This announcement contains translations of certain RMB amounts into U.S. dollars (“USD”) at specified rates solely for the convenience of the reader. Unless otherwise stated, all translations from RMB to USD were made at the rate of RMB6.8755 to US$1.00, the noon buying rate in effect on December 31, 2018 in the H.10 statistical release of the Federal Reserve Board. The Company makes no representation that the RMB or USD amounts referred could be converted into USD or RMB, as the case may be, at any particular rate or at all. For analytical presentation, all percentages are calculated using the numbers presented in the financial statements contained in this earnings release.

Statement Regarding Unaudited Condensed Financial Information

The unaudited financial information set forth above is preliminary and subject to potential adjustments. Adjustments to the consolidated financial statements may be identified when audit work has been performed for the Company’s year-end audit, which could result in significant differences from this preliminary unaudited condensed financial information.

About 21Vianet

21Vianet Group, Inc. is a leading carrier- and cloud-neutral Internet data center services provider in China. 21Vianet provides hosting and related services, including IDC services, cloud services, and business VPN services to improve the reliability, security and speed of its customers’ Internet infrastructure. Customers may locate their servers and equipment in 21Vianet’s data centers and connect to China’s Internet backbone. 21Vianet operates in more than 30 cities throughout China, servicing a diversified and loyal base of nearly 5,000 hosting and related enterprise customers that span numerous industries ranging from Internet companies to government entities and blue-chip enterprises to small- to mid-sized enterprises.

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Safe Harbor Statement

This announcement contains forward-looking statements. These forward-looking statements are made under the “safe harbor” provisions of the U.S. Private Securities Litigation Reform Act of 1995. These statements can be identified by terminology such as “will,” “expects,” “anticipates,” “future,” “intends,” “plans,” “believes,” “estimates” and similar statements. Among other things, quotations from management in this announcement as well as 21Vianet’s strategic and operational plans contain forward-looking statements. 21Vianet may also make written or oral forward-looking statements in its reports filed with, or furnished to, the U.S. Securities and Exchange Commission, in its annual reports to shareholders, in press releases and other written materials and in oral statements made by its officers, directors or employees to third parties. Statements that are not historical facts, including statements about 21Vianet’s beliefs and expectations, are forward-looking statements. Forward-looking statements involve inherent risks and uncertainties. A number of factors could cause actual results to differ materially from those contained in any forward-looking statement, including but not limited to the following: 21Vianet’s goals and strategies; 21Vianet’s expansion plans; the expected growth of the data center services market; expectations regarding demand for, and market acceptance of, 21Vianet’s services; 21Vianet’s expectations regarding keeping and strengthening its relationships with customers; 21Vianet’s plans to invest in research and development to enhance its solution and service offerings; and general economic and business conditions in the regions where 21Vianet provides solutions and services. Further information regarding these and other risks is included in 21Vianet’s reports filed with, or furnished to, the Securities and Exchange Commission. All information provided in this press release and in the attachments is as of the date of this press release, and 21Vianet undertakes no duty to update such information, except as required under applicable law.

Investor Relations Contacts:

21Vianet Group, Inc.

Rene Jiang

+86 10 8456 2121

IR@21Vianet.com

Julia Jiang

+86 10 8456 2121

IR@21Vianet.com

ICR, Inc.

Jack Wang

+1 (646) 405-4922

IR@21Vianet.com

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21VIANET GROUP, INC.

CONSOLIDATED BALANCE SHEETS

(Amount in thousands of Renminbi (“RMB”) and US dollars (“US$”))

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21VIANET GROUP, INC.

CONSOLIDATED BALANCE SHEETS

(Amount in thousands of Renminbi (“RMB”) and US dollars (“US$”))

10

21VIANET GROUP, INC.

CONSOLIDATED STATEMENTS OF OPERATIONS

(Amount in thousands of Renminbi (“RMB”) and US dollars (“US$”) except for number of shares and per share data)

11

21VIANET GROUP, INC.

CONSOLIDATED STATEMENTS OF OPERATIONS

(Amount in thousands of Renminbi (“RMB”) and US dollars (“US$”) except for number of shares and per share data)

12

21VIANET GROUP, INC.

RECONCILIATIONS OF GAAP AND NON-GAAP RESULTS

(Amount in thousands of Renminbi (“RMB”) and US dollars (“US$”))

13

21VIANET GROUP, INC.

SUPPLEMENTARY DISCLOSURE FOR HOSTING AND RELATED SERVICES

(Amount in thousands of Renminbi (“RMB”) and US dollars (“US$”))

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21VIANET GROUP, INC.

CONDENSED CONSOLIDATED STATEMENT OF CASH FLOWS

(Amount in thousands of Renminbi (“RMB”) and US dollars (“US$”))

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21VIANET GROUP, INC.

CONDENSED CONSOLIDATED STATEMENT OF CASH FLOWS

(Amount in thousands of Renminbi (“RMB”) and US dollars (“US$”))

Notes:

The Company adopted Accounting Standards Update (“ASU”) No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash on January 1, 2018 and retrospectively adjusted the condensed consolidated statement of cash flows for the three months ended December 31, 2017 by excluding the movement of restricted cash of RMB1,619.3 million.

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