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 November 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.

                     21Vianet Group, Inc.

Date: November 23, 2018

Exhibit Index

Exhibit 99.1 — Press Release

Exhibit 99.1

21Vianet Group, Inc. Reports Unaudited Third Quarter 2018 Financial Results

3Q18 adjusted EBITDA margin expanded to 28.2%

Raising full year 2018 adjusted EBITDA guidance

BEIJING, November 20, 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 third quarter ended September 30, 2018. The Company will hold a conference call at 8:00 pm on Tuesday, November 20, 2018, U.S. Eastern Time to discuss the financial results. Dial-in details are provided at the end of this release.

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

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

Third Quarter 2018 Operational Highlights

Mr. Alvin Wang, Chief Executive Officer and President of the Company, stated, “We maintained our strong momentum in revenue growth and profitability expansion during the third quarter of 2018. Our revenues grew by 14.6% year over year as we won new orders from existing customers, attracted additional notable customers, and experienced growth of Microsoft cloud service in China. Meanwhile, our adjusted EBITDA margin expanded to 28.2%, as we continuously grew our scale and improved our operational efficiency. While remain cautiously optimistic about our growth prospect and profitability against a backdrop of macroeconomic uncertainties, we will continue to execute our long-term strategy by accelerating our capacity growth in Tier 1 cities, nearby satellite cities and quasi-tier 1 cities. As we maintain our focus on sharpening our competitive edges in network quality and technology capability, we will be well-positioned to capitalize on the increasing market demand in China going forward.”

Ms. Sharon Liu, Chief Financial Officer of the Company, commented, “We once again delivered a solid quarter with strong top- and bottom-line growths. In the third quarter of 2018, our net revenues reported RMB870.1 million, beating the high-end of our guidance. More importantly, our adjusted EBITDA margin further increased to 28.2%, showcasing constant improvement in the past three quarters. We are raising our full year 2018 adjusted EBITDA guidance to reflect our achievement.”

Third 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 14.6% to RMB 870.1 million (US$ 126.7 million) in the third quarter of 2018 from RMB 759.3 million in the same period of 2017 and increased by 5.0% from RMB 828.3 million in the second quarter of 2018. The increase was primarily due to the growing demand for data centers and cloud services in the domestic market.

GROSS PROFIT: Gross profit increased by 11.1% to RMB 241.2 million (US$ 35.1 million) in the third quarter of 2018 from RMB 217.1 million in the same period of 2017 and increased by 5.1% from RMB 229.4 million in the second quarter of 2018. Gross margin decreased slightly to 27.7% in the third quarter of 2018 from 28.6% in the same period of 2017 but remained stable compared to the second quarter of 2018. The year-over-year decrease in gross margin was mainly due to an increase in depreciation.

ADJUSTED CASH GROSS PROFIT, which excludes depreciation, amortization, and share-based compensation expenses, increased by 24.2% to RMB 391.9 million (US$ 57.1 million) in the third quarter of 2018 from RMB315.6 million in the same period of 2017 and increased by 7.7% from RMB364.0 million in the second quarter of 2018. Adjusted cash gross margin expanded to 45.0% in the third quarter of 2018 from 41.6% in the same period of 2017 and 43.9% in the second quarter of 2018.

OPERATING EXPENSES: Total operating expenses increased by 1.1% to RMB 176.6 million (US$ 25.7 million) in the third quarter of 2018 from RMB174.6 million in the same period of 2017 but decreased by 0.8% from RMB177.9 million in the second quarter of 2018. As a percentage of net revenues, total operating expenses decreased to 20.3% in the third quarter of 2018 from 23.0% in the same period of 2017 and 21.5% in the second 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 3.0% to RMB 162.9 million (US$ 23.7 million) in the third quarter of 2018 from RMB158.1 million in the same period of 2017 and increased by 0.6% from RMB161.9 million in the second quarter of 2018. As a percentage of net revenues, adjusted operating expenses decreased to 18.7% in the third quarter of 2018 from 20.8% in the same period of 2017 and 19.5% in the second quarter of 2018. The decrease of adjusted operating expenses as a percentage of total revenues was primarily due to the successful implementation of the Company’s efficiency enhancement initiatives.

Sales and marketing expenses were RMB 39.9 million (US$ 5.8 million) in the third quarter of 2018 compared to RMB50.1 million in the same period of 2017 and from RMB41.8 million in the second quarter of 2018.

Research and development expenses were RMB 24.3 million (US$ 3.5 million) in the third quarter of 2018 compared to RMB22.2 million in the same period of 2017 and RMB22.2 million in the second quarter of 2018.

General and administrative expenses were RMB 110.2 million (US$ 16.1 million) in the third quarter of 2018 compared to RMB98.8 million in the same period of 2017 and RMB109.1 million in the second quarter of 2018.

ADJUSTED EBITDA: Adjusted EBITDA in the third quarter of 2018 increased by 39.5% to RMB 245.2 million (US$ 35.7 million) from RMB175.8 million in the same period of 2017 and increased by 10.9% from RMB221.1 million in the second quarter of 2018. Adjusted EBITDA in the third quarter of 2018 excludes share-based compensation expenses of RMB 12.9 million (US$ 1.9 million) and changes in the fair value of contingent purchase consideration payable, which was a loss of RMB 1.4 million (US$ 0.2 million). Adjusted EBITDA margin expanded to 28.2% in the third quarter of 2018 from 23.1% in the same period of 2017 and 26.7% in the second quarter of 2018.

NET LOSS: Net loss in the third quarter of 2018 was RMB 27.9 million (US$ 4.1 million) compared to a net loss of RMB1,479.1 million in the same period of 2017 and a net loss of RMB95.5 million in the second quarter of 2018. Net loss in the third quarter of 2018 included a foreign exchange loss of RMB55.0 million (US$8.0 million) compared to RMB5.6 million in the same period of 2017 and RMB73.4 million in the second quarter of 2018.

LOSS PER SHARE: Basic and diluted loss per share was RMB 0.04 (US$ 0.01) in the third quarter of 2018, which represents the equivalent of RMB 0.24 (US$ 0.06) 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 September 30, 2018, the Company’s cash and cash equivalents, restricted cash and short-term investments were RMB 2.96 billion (US$ 431.4 million).

Net cash generated from operating activities was RMB 260.7 million (US$ 38.0 million) in the third quarter of 2018 compared to RMB 206.6 million in the same period of 2017 and RMB 111.4 million in the second quarter of 2018.

Financial Outlook

For the fourth quarter of 2018, the Company expects net revenues to be in the range of RMB 870 million to RMB 890 million. Adjusted EBITDA is expected to be in the range of RMB 245 million to RMB 265 million.

Consequently, for the full year of 2018, the Company now expects net revenues to be in the range of RMB 3,370 million to RMB 3,390 million. Adjusted EBITDA is expected to be in the range of RMB 905 million to RMB 925 million. The midpoints of the Company’s updated estimates imply an increase of 13.6% year-over-year in total revenues and 36.4% 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 Tuesday, November 20, 2018 U.S. Eastern Time, or 9:00 am on Wednesday, November 21, 2018 Beijing Time, to discuss the financial results.

Participants may access the call by dialing the following numbers:

The replay will be accessible through November 28, 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 RMB 6.868 to US$1.00, the noon buying rate in effect on September 30, 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 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 September 30, 2017 by excluding the movement of restricted cash of RMB53.1 million.