Sasebo (佐世保市, Sasebo-shi) is a core city located in Nagasaki Prefecture, Japan.

The west, southwest, and southern parts of the city face the sea, and the east borders Saga Prefecture.

Because the mountains approach the central downtown area in the center of the city, bicycles are not widely used.

Rainy season lasts from early June to mid-July, and the summer is hot and humid.

It was a small fishing village under the control of nearby Hirado Domain until shortly after the start of the Meiji period.

Imperial Japanese Navy Admiral Tōgō Heihachirō, when surveying the coasts of northwestern Kyūshū for the site of a navy base, selected this location based on its protected, deep-water harbor, geographic proximity to China and Korea, and the presence of nearby coal fields.

After the end of the war, part of the base facilities were taken over by the United States Navy, which formed U.S. Fleet Activities Sasebo.

The city had 206,000 inhabitants in 1945 and suffered severe damage by United States bombing on June 29, 1945, during World War II.

Sasebo has a mayor-council form of government with a directly elected mayor and a unicameral city council of 33 members.

In terms of national politics, the city is part of the Nagasaki 3rd district of the lower house of the Diet of Japan.