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 August 2018

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: August 20, 2018

Exhibit Index

Exhibit 99.1 — Press Release

Exhibit 99.1

21Vianet Group, Inc. Reports Unaudited Second Quarter

2018 Financial Results

Adjusted EBITDA up 103.6% YoY to RMB221.1 million

Adjusted EBITDA margin expanded to 26.7%

Raised full year guidance for both net revenues and adjusted EBITDA

BEIJING, August 17, 2018 (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 second quarter ended June 30, 2018. The Company will hold a conference call at 8:00 pm on Thursday, August 16, 2018, U.S. Eastern Time to discuss the financial results. Dial-in details are provided at the end of this release.

Second Quarter 2018 Financial Highlights (including hosting and related services & MNS¹ business)

The financial results of the same period 2017 included those from both the hosting and related services business and the MNS business. The year-over-year improvement was partially driven by the disposal of the MNS business in September 2017.

Second Quarter 2018 Operational Highlights

Mr. Alvin Wang, Chief Executive Officer and President of the Company, stated, “Our strategy of focusing entirely on our core hosting and related services business continued to drive revenue and profitability growth. During the second quarter, we further improved our operational efficiency and achieved an adjusted EBITDA margin of 26.7%. Also we expanded our data center capacity, garnered new significant client wins in the Internet and financial services sectors, and expanded the order size from our existing clients. In addition, we have methodically entered into the promising wholesale data center market. We are confident that our commitment to network safety, availability, reliability, neutrality, and quality shall enable us to continuously gain market share and solidify our industry leadership.”

Ms. Sharon Liu, Chief Financial Officer of the Company, commented, “During the second quarter of 2018, we sustained solid growth in revenue and continued rapid improvement in adjusted EBITDA margin. Our net revenues from hosting and related services increased by 11.4% year over year to RMB828.3 million, while our adjusted EBITDA grew by 29.1% to RMB221.1 million. As we keep close track of our business development and maintain stringent internal control, we have been able to achieve or exceed our own guidance three quarters in a row. For the remainder of 2018, we expect continuous improvement in economy of scale and operational efficiency to result in EBITDA growth outpacing revenue growth and EBITDA margin expanding further.”

Second Quarter 2018 Financial Results

To fully reflect the Company’s performance, all analysis between “REVENUES” and “ADJUSTED EBITDA” presents 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 11.4% to RMB828.3 million (US$125.2 million) in the second quarter of 2018 from RMB743.4 million in the same period of 2017 and increased by 3.4% from RMB800.8 million in the first quarter of 2018. The increase was primarily due to continuously increasing demand from the Company’s new and existing customers.

GROSS PROFIT: Gross profit increased by 3.6% to RMB229.4 million (US$34.7 million) in the second quarter of 2018 from RMB221.4 million in the same period of 2017 and increased by 0.7% from RMB227.9 million in the first quarter of 2018. Gross margin decreased slightly to 27.7% in the second quarter of 2018 from 29.8% in the same period of 2017 but remained relatively stable compared to the first quarter of 2018. The decrease was mainly due to an increase in depreciation.

ADJUSTED CASH GROSS PROFIT, which excludes depreciation, amortization, and share-based compensation expenses, increased by 14.4% to RMB364.0 million (US$55.0 million) in the second quarter of 2018 from RMB318.2 million in the same period of 2017 and increased by 4.8% from RMB347.5 million in the first quarter of 2018. Adjusted cash gross margin expanded to 43.9% in the second quarter of 2018 from 42.8% in the same period of 2017 and 43.4% in the previous quarter. The increase was a result of the Company’s efficient management of cabinet capacity, power usage efficiency and human resources in the second quarter of 2018. Since the second quarter of 2018, the Company eliminated approximately 214 lower-margin cabinets in partnered data centers, bringing the total number of partnered cabinets to 4,982 in the second quarter of 2018 from 5,768 in the same period of 2017.

OPERATING EXPENSES: Total operating expenses increased by 3.0% to RMB177.9 million (US$26.9 million) in the second quarter of 2018 from RMB172.7 million in the same period of 2017 and increased by 3.8% from RMB171.5 million in the first quarter of 2018. As a percentage of net revenues, total operating expenses was 21.5% in the second quarter of 2018, compared to 23.2% in the same period of 2017 and 21.4% in the first quarter of 2018.

Adjusted operating expenses, which exclude share-based compensation expenses and changes in the fair value of contingent purchase consideration payable, increased by 1.2% to RMB161.9 million (US$24.5 million) in the second quarter of 2018 from RMB160.0 million in the same period of 2017 but decreased by 3.2% from RMB167.2 million in the first quarter of 2018. As a percentage of net revenues, adjusted operating expenses decreased to 19.5% in the second quarter of 2018 from 21.5% in the same period of 2017 and 20.9% in the previous quarter. The decrease was primarily due to the successful implementation and execution of the Company’s cost control measures.

Sales and marketing expenses were RMB41.8 million (US$6.3 million) in the second quarter of 2018, compared to RMB45.0 million in the same period of 2017 and from RMB41.2 million in the first quarter of 2018.

Research and development expenses were RMB22.2 million (US$3.3 million) in the second quarter of 2018 compared to RMB23.9 million in the same period of 2017 and RMB22.0 million in the first quarter of 2018.

General and administrative expenses were RMB109.1 million (US$16.5 million) in the second quarter of 2018 compared to RMB104.8 million in the same period of 2017 and RMB112.3 million in the first quarter of 2018.

ADJUSTED EBITDA: Adjusted EBITDA for the second quarter of 2018 increased by 29.1% to RMB221.1 million (US$33.4 million) from RMB171.3 million in the same period of 2017 and increased by 12.8% compared to the first quarter of 2018. Adjusted EBITDA for the second quarter of 2018 excludes share-based compensation expenses of RMB10.8 million (US$1.6 million) and changes in the fair value of contingent purchase consideration payable, which was a loss of RMB5.5 million (US$0.8 million). Adjusted EBITDA margin expanded to 26.7% in the second quarter of 2018 from 23.0% in the same period of 2017 and 24.5% in the first quarter of 2018.

NET PROFIT/LOSS: Net loss for the second quarter of 2018 was RMB95.5 million (US$14.4 million) compared to net loss of RMB119.3 million in the same period of 2017 and net profit of RMB34.7 million in the first quarter of 2018. The quarter-over-quarter change was mainly due to foreign exchange fluctuation.

PROFIT/LOSS PER SHARE: Basic and diluted loss per share was RMB0.14 (US$0.02) in the second quarter of 2018, which represents the equivalent of RMB0.84 (US$0.12) per American Depositary Share (“ADS”). Each ADS represents six ordinary shares. Diluted profit per share is calculated using net profit divided by the weighted average number of shares.

As of June 30, 2018, the Company’s cash and cash equivalents, restricted cash and short-term investments were RMB2.66 billion (US$401.6 million).

Net cash generated from operating activities was RMB111.4 million (US$16.8 million) in the second quarter of 2018.

Recent Developments

In August 2018, the Company secured over 1,000 datacenter cabinets in Shanghai, China. The newly added cabinets are consistent with the Company’s capacity expansion strategy. The Company expects these additional cabinets to be delivered in two batches by the end of 2018.

In August 2018, the Company and a subsidiary of Tus Holdings entered into a long-term lease agreement for the Company’s rights to use over 20,000m² property in Beijing, China. The new property is expected to add 3,000 to 4,000 additional cabinet to the Company’s datacenter network. The first batch of cabinets is estimated to be delivered in the second half of 2019.

On July 19, 2018, the Company announced that its wholly-owned subsidiary Shanghai Blue Cloud Technology Co., Ltd. (“21V Blue Cloud”) has entered into distribution agreements with four world-class cloud service providers including Unify Cloud, AvePoint, Agile Point and Fadada.com who officially authorized 21V Blue Cloud as a distributor of their products and services in mainland China.

Financial Outlook

For the third quarter of 2018, the Company expects net revenues to be in the range of RMB840 million to RMB860 million. Adjusted EBITDA is expected to be in the range of RMB230 million to RMB250 million.

Based on solid first half 2018 results, the Company is raising its full year 2018 guidance for both net revenues and adjusted EBITDA. For the full year, the Company now expects net revenues to be in the range of RMB3.28 billion to RMB3.38 billion. Adjusted EBITDA for the full year is expected to be in the range of RMB800 million to RMB880 million.

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 Thursday, August 16, 2018 U.S. Eastern Time, or 8:00 am on Friday, August 17, 2018 Beijing Time, to discuss the financial results.

Participants may access the call by dialing the following numbers:

The replay will be accessible through August 24, 2018 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.

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 RMB6.6171 to US$1.00, the noon buying rate in effect on June 29, 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, cloud services, and business VPN services, improving the reliability, security and speed of its customers’ Internet infrastructure. Customers may locate their servers and networking equipment in 21Vianet’s data centers and connect to China’s Internet backbone through 21Vianet’s extensive fiber optic network. 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.

SUPPLEMENTARY DISCLOSURE FOR HOSTING AND RELATED SERVICES

(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$”))

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 June 30, 2017 by excluding the movement of restricted cash of RMB150.5 million.