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 May 2019

Commission File Number: 001-35126

21Vianet Group, Inc.

Guanjie Building, Southeast 1st Floor, 10# 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: May 17, 2019

Exhibit Index

Exhibit 99.1 — Press Release

Exhibit 99.1

21Vianet Group, Inc. Reports Unaudited First Quarter 2019 Financial Results

BEIJING, May 17, 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 first quarter ended March 31, 2019. The Company will hold a conference call at 8:00 P.M. on Thursday, May 16, 2019, U.S. Eastern Time to discuss the financial results. Dial-in details are provided at the end of this release.

First Quarter 2019 Financial Highlights

First Quarter 2019 Operational Highlights

Mr. Alvin Wang, Chief Executive Officer and President of the Company, stated, “In the first quarter, we delivered a stable and healthy performance. In May, we made exciting progress in the expansion of our cloud solution offerings through our partnership with Microsoft to launch their Dynamic 365 cloud service. We have also been strategically growing our resource pipeline in anticipation of our corporate clients’ increasing demand for scalable data hosting solutions, as demonstrated by our recent acquisition in Beijing. Looking to the future, we have created a three-year growth plan, specifically designed to meet the increasing market demands from medium- to large-scale customers and ensure the efficient allocation of resources. Increasing our cabinet capacity to service large-scale corporate clients, one of the plan’s key focuses, will help us to support customer growth, broaden our customer base, and provide shareholders a higher return on their investments.”

Ms. Sharon Liu, Chief Financial Officer of the Company, commented, “In the first quarter, our net revenues met our previous target and increased to RMB871.9 million. Furthermore, despite customer churn caused by business restructurings, our adjusted EBITDA exceeded the high end of our guidance, increasing by 29.3% year over year. As a result, our adjusted EBITDA margin expanded to 29.1%. In April, we also enhanced our cash position by issuing US$300 million senior notes due 2021. We are confident that a stronger balance sheet will help to accelerate growth moving into the quarters ahead.”

Three-Year Growth Plan

In order to capitalize on forecasted market trends and augment its market position as a reliable carrier- and cloud-neutral Internet data center services provider in China, the Company has developed a Three-Year Growth Plan, outlining objectives from 2019 to 2021. The plan includes targets for capacity expansion, year-over-year revenue growth rates, and year-over-year adjusted EBITDA growth rate:

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

First Quarter 2019 Financial Results

REVENUES: Net revenues increased by 8.9% to RMB871.9 million (US$129.9 million) in the first quarter of 2019 from RMB800.8 million in the same period of 2018 and decreased by 3.3% from RMB901.9 million in the fourth quarter of 2018. This year-over-year increase was primarily attributable to the growing demand for data centers and cloud services in the domestic market, partially offset by the churn of a major customer. The sequential decrease was mostly due to customer churn, related to a major client’s business restructuring and the seasonality effect in non-recurring revenue.

GROSS PROFIT: Gross profit increased by 5.6% to RMB240.8 million (US$35.9 million) in the first quarter of 2019 from RMB227.9 million in the same period of 2018 and decreased by 2.3% from RMB246.3 million in the fourth quarter of 2018. Gross margin was 27.6% in the first quarter of 2019 compared to 28.5% in the same period of 2018 and 27.3% in the fourth quarter of 2018.

ADJUSTED CASH GROSS PROFIT, which excludes depreciation, amortization, and share-based compensation expenses, increased by 17.0% to RMB406.7 million (US$60.6 million) in the first quarter of 2019 from RMB347.5 million in the same period of 2018 and decreased by 0.6% from RMB409.2 million in the fourth quarter of 2018. Adjusted cash gross margin expanded to 46.6% in the first quarter of 2019 from 43.4% in the same period of 2018 and 45.4% in the fourth quarter of 2018. The year-over-year improvement in adjusted cash gross margin was mainly attributable to the Company’s continuous efforts in maximizing its operating efficiency.

OPERATING EXPENSES: Total operating expenses increased by 9.3% to RMB187.5 million (US$27.9 million) in the first quarter of 2019 from RMB171.5 million in the same period of 2018 and increased by 3.3% from RMB181.4 million in the fourth quarter of 2018. As a percentage of net revenues, total operating expenses increased slightly to 21.5% in the first quarter of 2019 from 21.4% in the same period of 2018 and 20.1% in the fourth quarter of 2018. The increase of operating expenses as a percentage of net revenues was primarily due to the slowdown in revenue growth caused by customer churn.

Sales and marketing expenses were RMB44.1 million (US$6.6 million) in the first quarter of 2019, an increase of 6.9% from RMB41.2 million in the same period of 2018 and a decrease of 10.4% from RMB49.2 million in the fourth quarter of 2018. The year-over-year increase was mainly attributable to increased marketing activities and higher sales commissions. The increase was in line with the year-over-year increase of the Company’s net revenues in the first quarter of 2019.

Research and development expenses were RMB22.6 million (US$3.4 million) in the first quarter of 2019 compared to RMB22.0 million in the same period of 2018 and RMB23.6 million in the fourth quarter of 2018.

General and administrative expenses were RMB120.8 million (US$18.0 million) in the first quarter of 2019 compared to RMB112.3 million in the same period of 2018 and RMB131.0 million in the fourth quarter of 2018. The year-over-year increase was mainly attributable to share-based compensation expenses that the Company recognized during the first quarter of 2019.

ADJUSTED OPERATING EXPENSES, which exclude share-based compensation expenses and changes in the fair value of contingent purchase consideration payables, increased by 2.5% to RMB171.3 million (US$25.5 million) in the first quarter of 2019 from RMB167.2 million in the same period of 2018 and decreased by 0.7% from RMB172.4 million in the fourth quarter of 2018. As a percentage of net revenues, adjusted operating expenses decreased to 19.6% in the first quarter of 2019 from 20.9% in the same period of 2018 and increased slightly from 19.1% in the fourth quarter of 2018.

ADJUSTED EBITDA: Adjusted EBITDA in the first quarter of 2019 increased by 29.3% to RMB253.5 million (US$37.8 million) from RMB196.0 million in the same period of 2018 and decreased by 0.7% from RMB255.3 million in the fourth quarter of 2018. Adjusted EBITDA in the first quarter of 2019 excluded share-based compensation expenses of RMB16.6 million (US$2.5 million). Adjusted EBITDA margin expanded to 29.1% in the first quarter of 2019 from 24.5% in the same period of 2018 and 28.3% in the fourth quarter of 2018.

NET PROFIT/LOSS: Net profit attributable to ordinary shareholders in the first quarter of 2019 was RMB5.6 million (US$0.8 million) compared to a net profit of RMB32.8 million in the same period of 2018 and a net loss of RMB114.1 million in the fourth quarter of 2018. Net profit attributable to ordinary shareholders in the first quarter of 2019 included a foreign exchange gain of RMB29.5 million (US$4.4 million) compared to RMB44.8 million in the same period of 2018 and RMB2.5 million in the fourth quarter of 2018.

PROFIT/LOSS PER SHARE: Basic and diluted profit per share were RMB0.01 (US$0.1 cent) in the first quarter of 2019, which represents the equivalent of RMB0.06 (US$0.6 cent) 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 March 31, 2019, the Company’s cash and cash equivalents, restricted cash, and short-term investments were RMB2.54 billion (US$379.0 million).

Net cash generated from operating activities was RMB32.4 million (US$4.8 million) in the first quarter of 2019 compared to RMB95.9 million in the same period of 2018 and RMB237.0 million in the fourth quarter of 2018.

Recent Development

On April 10, 2019, the Company announced that it priced the offering of US$300.0 million in aggregate principal amount of the USD-denominated notes due 2021 at an interest rate of 7.875% per annum, following a successful tender offer by the Company of the outstanding US$300.0 million 7.000% senior notes due 2020 (the “2020 Notes”). Approximately US$150.8 million in principal amount of the 2020 Notes was validly tendered, representing approximately 50.3% of the US$300.0 million total aggregate principal amount outstanding.

On May 6, 2019, the Company facilitated the official launch of Microsoft’s cloud service Dynamics 365 in China. Dynamics 365’s launch marked the completion of Microsoft’s intelligent cloud launch in the Chinese market. As of launch, the Company is Microsoft’s local partner for all of its three major cloud offerings: Microsoft Azure, Office 365, and Dynamics 365.

The Company recently reached an agreement to make an acquisition in Beijing to both satisfy increasing customer demand in the southern part of Beijing and support the Company’s continuing expansion plans in Beijing. This acquisition is expected to deliver approximately 1,000 cabinets by the middle of 2019 and has the potential for further expansion.

Financial Outlook

For the second quarter of 2019, the Company expects net revenues to be in the range of RMB880 million to RMB900 million. Adjusted EBITDA is expected to be in the range of RMB250 million to RMB270 million.

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 P.M. on Thursday, May 16, 2019, U.S. Eastern Time, or 8:00 A.M. on Friday, May 17, 2019, Beijing Time, to discuss the financial results.

Participants may access the call by dialing the following numbers:

The replay will be accessible through May 24, 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 a 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.

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 RMB6.7112 to US$1.00, the noon buying rate in effect on March 29, 2019, 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.

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

21VIANET GROUP, INC.

CONSOLIDATED BALANCE SHEETS

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

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)

21VIANET GROUP, INC.

RECONCILIATIONS OF GAAP AND NON-GAAP RESULTS

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

21VIANET GROUP, INC.

CONDENSED CONSOLIDATED STATEMENT OF CASH FLOWS

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