The Gulf of Tonkin (480 kmx240 km) lies between Vietnam and China. Notably shallow (less than 60 meters deep), it is the northwest arm of the South China Sea. The Red River flows into the Gulf. Haiphong, Vietnam, and Beihai, China, are the chief ports. The Chinese Hainan Island lies in the Gulf. Other small islands in the gulf include:
- Weizhou, China
Because in several Asian languages, "Tonkin" means both Tonkin and Tokyo, Vietnamese call it the Vịnh Bắc Bộ ("Bac Bo Gulf"). Modern Chinese geographers use this convention as well, calling it the Beibu Gulf (北部灣).