The former Chinese placename was Tsu-lo-san (Chinese: 諸羅山; pinyin: Zhūluóshān; Pe̍h-ōe-jī: Chu-lô-san), a representation of the original Formosan-language name Tirosen. A shortened version, Tsulo, was then used to name Tsulo County, which originally covered the underdeveloped northern two-thirds of the island. In 1704, the county seat was moved to Tsulosan, the site of modern-day Chiayi City. Following the 1723 Zhu Yigui rebellion, the county was reduced in size. In 1787, after the Lin Shuang-wen Rebellion (林爽文事件), the county and city were renamed Kagi.
In December 1978, the Executive Yuan approved the upgrade of Chiayi City to become a provincial city, thus in December 1981, Chiayi County government relocated the county capital to Dongshiliao Farm in Taibao Township and Chiayi City finally became a provincial city in July 1982.
In March 1989, Wufong Township was renamed to Alishan Township and in July 1991, Taibao Township was restructured to become Taibao City. In November 1991, Chiayi County government relocated the county capital from Dongshiliao Farm to Hsiangho New Village in Taibao City.PuziTownship was restructured to a county-controlled city in September 1992.
Chiayi County is bordered with Mount Yu to the east, Taiwan Strait to the west, Tainan City to the south and Yunlin County to the north. It spans over 1,903 km2, about 5.35% of Taiwan area. Due to its location along the Tropic of Cancer path, Chiayi County has a unique landscape with other regions in Taiwan. In most places, most of them have either desserts or semi-desserts.
The current population of Chiayi County as of December 2014 is 524,783 people. The county has been experiencing a population decline since 2009 due to higher migration out of the county and higher death rate than birth rate.