Saikai (西海市, Saikai-shi) is a city located in Nagasaki Prefecture, Japan.

The highest peak on the peninsula is Mount Nagaura, 561 meters above sea level (the summit is in Nagasaki City).

Nagasaki Prefecture Saikai has a humid subtropical climate (Köppen:Cfa) with hot summers and cool winters.

Saikai has a mayor-council form of government with a directly elected mayor and a unicameral city council of 18 members.

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

However, on Oshima island, north of Saikai and close to Sasebo, a large shipyard is active building bulk ships and metal structures, i.e. Oshima Shipbuilding, in which the Sumitomo Heavy Industries has shares.