The East China Sea is a marginal sea and part of the Pacific Ocean, surrounded by China, Japan, South Korea and Taiwan. Its surface area is around 1,200,000 km². In China, the sea is called the East Sea. In South Korea, it is sometimes called the South Sea.

Its northern part, between China and the Korean peninsula is called the Yellow Sea. The Sea of Japan is connected with it through Tsushima Strait (Korea Strait).

It is a place of territorial disputes between the PRC (People's Republic of China) and Japan, notably for undersea oil resources. In October to November 2004, Japan claimed that a PRC submarine entered its EEZ. The PRC claims that disputed ocean territory as its own due to its being part of PRC's continental shelf, while Japan claims the disputed ocean territory as its own because its within 200 nautical miles from Japan's cost.

The East China Sea is in the centre of dispute recently due to the discovery of a Gas Field which lies on the Japan's claimed border. While China claim the field is completely within its own territory, according to its version of maritime border, Japan is trying to seek a share of the field, saying part of the gas field is within its territory, according to its version of maritime border, with the recent approval of civilian research of the field.

See also: Xihu Trough, Geography of China