Haikou is the capital and most populous city of the Chinese province of Hainan. Haikou city is situated on the northern coast of Hainan, by the mouth of the Nandu River. The northern part of the city is on the 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 four urban districts of the city.

Haikou was originally a port city, serving as the port for Qiongshan. During the Chinese Civil War, Haikou was one of the last Nationalist strongholds to be taken by the Communists with the Battle of Hainan Island in 1950. Currently, more than half of the island's total trade still goes through Haikou's ports with the Temple of the Five Lords located to the southeast of the city.

The city is home to Hainan University, a comprehensive research university under the Project 211 and the Double First Class University Plan in certain disciplines, 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" – similar to Portsmouth in Britain.^{[citation needed]} Haikou originally served as the port for Qiongshan, the ancient administrative capital of Hainan island, located some 5 km (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. It was formerly known internationally as 'Hoihow', based on the Cantonese pronunciation by the British Raj officers. In 1926, Haikou overtook Qiongshan in population and it was declared a separate administrative city.

The city and island of Hainan stayed under the control of the Nationalists until April 1950, when it fell to the Communists during the Landing Operation on Hainan Island.

Since 1949, Haikou has maintained its position as Hainan's main port, handling more than half of the island's total trade. It has replaced Qiongshan as the island's administrative capital. In 1988, Haikou was made a prefecture-level city as well as the capital of the newly created Hainan Province.

Haikou old town contains a small portion of the oldest buildings in Haikou city that returnees from overseas built. The houses are a mixture of styles including Portuguese, French, and Southeast Asian. The streets used to be divided into different areas selling Chinese and western medicine, for silk and bespoke clothes, one for fresh fish and meat, and others for the sale of incense, candles, paper, ink, and other goods.

Various projects are currently under discussion to decide the best way to restore and preserve these historical buildings.^{[citation needed]}