The northwestern part is at the southeastern edge of the Kumamoto Plain, but most of the town area is included in the Kyushu Mountains and is at a high altitude.

After the Meiji restoration, the town of Kōsa and villages of Miyauchi, Tatsuno, Shirahata, and Otome were established with the creation of the modern municipalities system on April 1, 1889.

Kōsa has a mayor-council form of government with a directly elected mayor and a unicameral town council of 11 members.

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

Since its inception in 1983, the Kumamoto Kōsa 10-Miler road race has seen several top rated achievements being set during the event, which is run in December.