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 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: November 19, 2019

Exhibit Index

Exhibit 99.1 — Press Release

Exhibit 99.1

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

BEIJING, November 18, 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 third quarter ended September 30, 2019. The Company will hold a conference call at 8:00 P.M. on Monday, November 18, 2019, U.S. Eastern Time to discuss the financial results. Dial-in details are provided at the end of this release.

Third Quarter 2019 Financial Highlights

Third Quarter 2019 Operational Highlights

Mr. Alvin Wang, Chief Executive Officer and President of the Company, stated, “In the third quarter, both our revenues and adjusted EBITDA exceeded the high end of our previous guidance range. This strong performance was attributable to our elevated value propositions and increased partnerships across industry sectors. We also continued to refine our product offerings and expand our capacity pipeline in response to the increasing demands for scalable and dependable IT solutions amid the current network transformation. Such advances were in sync with our three-year growth plan, increasing the appeal of our offerings to large-scale partners like Alibaba and enabling us to explore new opportunities for cooperation with a diverse group of retail clients through our innovative services. With the right solutions, rich industry knowledge, and a highly-experienced team in place, we are confident in our ability to generate superior, sustainable shareholder value in the long term.”

Ms. Sharon Liu, Chief Financial Officer of the Company, commented, “During the third quarter, our revenues increased by 12.7% year over year and our adjusted EBITDA increased by 11.1% year over year, driven by the expanding scope of corporate digitalization across China. We also continued to actively engage in dialogues with our clients to remain at the vanguard of industry trends, better position ourselves to secure additional business opportunities, and ramp up our cabinet deliveries, which further contributed to our healthy cash position in the period.”

Third Quarter 2019 Financial Results

REVENUES: Net revenues increased by 12.7% to RMB981.0 million (US$137.2 million) in the third quarter of 2019 from RMB870.1 million in the same period of 2018 and increased by 10.5% from RMB888.0 million in the second quarter of 2019. The year-over-year increase was primarily attributable to the growing domestic demand for data centers, driven by the expanding scope of corporate digitalization across China.

GROSS PROFIT: Gross profit was RMB222.6 million (US$31.1 million) in the third quarter of 2019, compared to RMB241.2 million in the same period of 2018 and RMB228.2 million in the second quarter of 2019. Gross margin was 22.7% in the third quarter of 2019, compared to 27.7% in the same period of 2018 and 25.7% in the second quarter of 2019. The year-over-year decrease was mainly due to the introduction of certain lower-margin products and higher rent and utility costs, and partially due to the delivery of additional pipeline capacity.

ADJUSTED CASH GROSS PROFIT, which excludes depreciation, amortization, and share-based compensation expenses, increased by 1.2% to RMB396.7 million (US$55.5 million) in the third quarter of 2019 from RMB391.9 million in the same period of 2018 and decreased by 1.8% from RMB403.8 million in the second quarter of 2019. Adjusted cash gross margin was 40.4% in the third quarter of 2019, compared to 45.0% in the same period of 2018 and 45.5% in the second quarter of 2019.

OPERATING EXPENSES: Total operating expenses decreased by 11.0% to RMB157.1 million (US$22.0 million) in the third quarter of 2019 from RMB176.6 million in the same period of 2018 and decreased by 6.6% from RMB168.2 million in the second quarter of 2019. As a percentage of net revenues, total operating expenses reduced to 16.0% in the third quarter of 2019 from 20.3% in the same period of 2018 and 18.9% in the second quarter of 2019. The reduction of operating expenses as a percentage of net revenues was primarily due to the Company’s continuous efforts in maximizing its operating efficiency and operating leverage.

Sales and marketing expenses were RMB52.4 million (US$7.3 million) in the third quarter of 2019, an increase of 31.3% from RMB39.9 million in the same period of 2018 and an increase of 12.4% from RMB46.6 million in the second quarter of 2019. The increase of sales and marketing expenses was mainly attributable to increased marketing activities.

Research and development expenses were RMB22.5 million (US$3.2 million) in the third quarter of 2019 compared to RMB24.3 million in the same period of 2018 and RMB18.8 million in the second quarter of 2019.

General and administrative expenses were RMB82.2 million (US$11.5 million) in the third quarter of 2019 compared to RMB110.2 million in the same period of 2018 and RMB102.3 million in the second quarter of 2019. The decrease was mainly attributable to the Company’s continuous efforts in maximizing its operating efficiency.

ADJUSTED OPERATING EXPENSES, which exclude share-based compensation expenses and changes in the fair value of contingent purchase consideration payables, decreased by 10.2% to RMB146.2 million (US$20.5 million) in the third quarter of 2019 from RMB162.9 million in the same period of 2018 and decreased by 9.3% from RMB161.3 million in the second quarter of 2019. As a percentage of net revenues, adjusted operating expenses decreased to 14.9% in the third quarter of 2019 from 18.7% in the same period of 2018 and 18.2% in the second quarter of 2019.

ADJUSTED EBITDA: Adjusted EBITDA in the third quarter of 2019 increased by 11.1% to RMB272.5 million (US$38.1 million) from RMB245.2 million in the same period of 2018 and increased by 4.5% from RMB260.7 million in the second quarter of 2019. Adjusted EBITDA in the third quarter of 2019 excluded share-based compensation expenses of RMB11.3 million (US$1.6 million). Adjusted EBITDA margin was 27.8% in the third quarter of 2019, compared to 28.2% in the same period of 2018 and 29.4% in the second quarter of 2019.

NET PROFIT/LOSS: Net loss attributable to ordinary shareholders in the third quarter of 2019 was RMB69.5 million (US$9.7 million) compared to RMB29.6 million in the same period of 2018 and RMB102.1 million in the second quarter of 2019. Net loss attributable to ordinary shareholders in the third quarter of 2019 included a foreign exchange loss of RMB40.2 million (US$5.6 million) compared to RMB55.0 million in the same period of 2018 and RMB39.9 million in the second quarter of 2019, and an interest expense of RMB96.9 million (US$13.6 million) compared to RMB60.8 million in the same period of 2018 and RMB91.2 million in the second quarter of 2019.

PROFIT/LOSS PER SHARE: Basic and diluted loss per share were RMB0.10 (US$ 1 cent) in the third quarter of 2019, which represents the equivalent of RMB0.60 (US$6 cent) per American Depositary Share (“ADS”). Each ADS represents six ordinary shares. Diluted loss per share is calculated using net loss attributable to ordinary shareholders divided by the weighted average number of diluted shares outstanding.

As of September 30, 2019, the Company’s cash and cash equivalents, restricted cash, and short-term investments were RMB2.94 billion (US$411.9 million).

Net cash generated from operating activities was RMB103.0 million (US$14.4 million) in the third quarter of 2019 compared to RMB260.7 million in the same period of 2018 and RMB127.1 million in the second quarter of 2019.

Recent Development

The Company signed a memorandum of understanding (the “MoU”) with Alibaba Group (“Alibaba”) on October 14, 2019. As part of the MoU, the Company will deploy IDC services to support Alibaba in its expansion throughout Eastern China. The project will be deployed in two phases. The first phase expects to complete construction and cabinet deliveries by the first half of 2020. During the term of the project contract, the first phase is expected to generate revenue of RMB1.6 billion.

Financial Outlook

For the fourth quarter of 2019, the Company expects net revenues to be in the range of RMB1,030 million to RMB1,050 million. Adjusted EBITDA is expected to be in the range of RMB245 million to RMB265 million.

For the full year of 2019, the Company expects net revenues to be in the range of RMB3,771 million to RMB3,791 million. Adjusted EBITDA is expected to be in the range of RMB1,033 million to RMB1,053 million. The midpoints of the Company’s updated estimates imply an increase of 11.2% year over year in total revenues and an increase of 13.6% 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 Monday, November 18, 2019, U.S. Eastern Time, or 9:00 A.M. on Tuesday, November 19, 2019, Beijing Time, to discuss the financial results.

Participants may access the call by dialing the following numbers:

The replay will be accessible through November 26, 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 RMB7.1477 to US$1.00, the noon buying rate in effect on September 30, 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$”))