The companies included Baidu Inc., Tencent Holdings, Didi Chuxing, SoftBank, and a ByteDance-backed firm, the State Administration for Market Regulation (SAMR) said in a statement on Friday.
The companies were fined 500,000 yuan ($77,000) each for behavior that caused market concentration but did not exclude all competition from other companies, SAMR said.
Tencent said in a statement it would actively rectify operations and provide the regulator with timely reports on deals in future.
ByteDance said a joint venture between its affiliated firm and Shanghai Dongfang Newspaper Co. Ltd., which were both fined, was never in operation and the JV was disbanded in January.
SoftBank declined to comment. Baidu and Didi did not immediately respond to requests for comment.
China has stepped up scrutiny of its internet giants in recent months, citing concerns over monopolistic behavior and potential infringement of consumer rights.
The regulator has fined Alibaba, Tencent-backed China Literature and other firms for not reporting deals properly for anti-trust reviews. It also fined company involved in an auto-related deal on Thursday.
By Yingzhi Yang, Cheng Leng, Pei Li, Yilei Sun and Tony Munroe