[1] The total area of the city is 101.06 square kilometres (39.02 sq mi) Kainan is located in northern Wakayama prefecture and facing the Kii Channel.

Wakayama Prefecture Kainan has a Humid subtropical climate (Köppen Cfa) characterized by warm summers and cool winters with light to no snowfall.

The area of the modern city of Kainan was within ancient Kii Province, and Yayoi pottery and Kofun burial mounds indicate settlement since prehistoric times.

During the Sengoku period, the area was a battleground between the forces of Oda Nobunaga and the local Saika ikki, one of many Ikkō-ikki movements.

The village of Kuroe was founded with the creation of the modern municipalities system on April 1, 1889 and was raised to town status on July 7, 1896.

Kuroe merged with the neighboring towns of Utsumi and Hikata and the village of Ono to form the city of Kainan on May 1, 1934.