Kōsa (甲佐町, Kōsa-machi) is a town located in Kamimashiki District, Kumamoto Prefecture, Japan. As of 31 July 2024, the town had an estimated population of 10,004 in 4441 households, and a population density of 24 persons per km. The total area of the town is 57.93 km (22.37 sq mi).

Kōsa is located in the central inland region of Kumamoto Prefecture, about 20 kilometers southeast of Kumamoto City. 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. The Midorigawa River, a first-class river, flows through the center of the town.

Kumamoto Prefecture

• Kumamoto
• Mifune
• Misato
• Uki

Kōsa has a humid subtropical climate (Köppen climate classification Cfa) with hot, humid summers and cool winters. There is significant precipitation throughout the year, especially during June and July. The average annual temperature in Kōsa is 16.3 °C (61.3 °F). The average annual rainfall is 2,145.6 mm (84.47 in) with June as the wettest month. The temperatures are highest on average in August, at around 27.1 °C (80.8 °F), and lowest in January, at around 5.4 °C (41.7 °F). The highest temperature ever recorded in Kōsa was 37.9 °C (100.2 °F) on 14 August 2018; the coldest temperature ever recorded was −8.8 °C (16.2 °F) on 25 January 2016.

Per Japanese census data, the population of Kōsa in 2020 is 10,132 people. Kōsa has been conducting censuses since 1920.

Historical population

Year Pop. ±% 1920 13,880 —     1925 14,629 +5.4% 1930 15,457 +5.7% 1935 15,630 +1.1% 1940 15,314 −2.0% 1945 19,517 +27.4% 1950 19,915 +2.0% 1955 18,610 −6.6% 1960 16,765 −9.9% 1965 14,975 −10.7% 1970 13,620 −9.0%	Year Pop. ±% 1975 13,160 −3.4% 1980 12,989 −1.3% 1985 12,864 −1.0% 1990 12,459 −3.1% 1995 12,372 −0.7% 2000 12,012 −2.9% 2005 11,604 −3.4% 2010 11,180 −3.7% 2015 10,717 −4.1% 2020 10,132 −5.5%
Kōsa population statistics

The area of Kōsa was part of ancient Higo Province, During the Edo Period it was part of the holdings of Kumamoto Domain. 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. On January 1, 1955 Miyauchi, Tatsuno, Shirahata, and Otome merged with Kōsa.

Kōsa has a mayor-council form of government with a directly elected mayor and a unicameral town council of 11 members. Kōsa, collectively with the other municipalities of Kamimashiki District contributes two members to the Kumamoto Prefectural Assembly. In terms of national politics, the town is part of the Kumamoto 3rd district of the lower house of the Diet of Japan.

The local economy relies on agriculture, and Kōsa's geographic position as a logistics hub.

Kōsa has four public elementary schools and one public junior high school operated by the town government, and one public high schools operated by the Kumamoto Prefectural Board of Education.

Kōsa does not have any passenger railway service. The nearest station is Matsubase Station on the [[[JR Kyushu]] station is Matsubase Station on the Kagoshima Main Line in neighboring Uki.

• Kyushu Expressway
• National Route 443

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. The most recent new world record for the 10 miles was set in December 2022 by Bernard Koech.

• Media related to Kōsa, Kumamoto at Wikimedia Commons
• Kōsa official website (in Japanese)