Hǎikǒu (Chinese: 海口), is the capital and most populous city of Hainan province, People's Republic of China. It is situated on the northern coast of Hainan, by the mouth of the Nandu River. The northern part of the city is the district of Haidian Island, which is separated from the main part of Haikou by the Haidian River, a branch of the Nandu.
Administratively, Haikou is a prefecture-level city, comprising four districts, and covering 2,280 square kilometres (880 sq mi). There are 2,046,189 inhabitants in the built up area all living within the 4 urban districts of the city.
Haikou was originally a port city. Today, more than half of the island's total trade still goes through its ports. The city is home to Hainan University, which has its main campus on Haidian Island.
The hanzi characters comprising the city's name, 海口, mean ocean/sea and mouth/port, respectively. Thus, the name "Haikou" is also a word for "seaport". Haikou originally served as the port for Qiongshan, the ancient administrative capital of Hainan island, located some 5 kilometres (3.1 mi) inland to the south east. During its early history Haikou was a part of Guangdong province. In the 13th century it was fortified and became a military post under the Ming dynasty (1368–1644). The port is located west of the mouth of the Nandu River, Hainan's principal river. When Qiongshan was opened to foreign trade under the Treaty of Tianjin in 1858, Haikou started to rival the old administrative city. In 1926, Haikou overtook Qiongshan in population and it was declared a separate administrative city. Haikou was developed as a port during the Sino-Japanese War (1937–45) when the Japanese invaded and occupied Hainan Island from early 1939 to 1945.