was a town located in Ito District, Wakayama Prefecture, Japan.
As of 2003, the town had an estimated population of 14,889 and a density of 742.22 persons per km<sup>2</sup>. The total area was 20.06 km<sup>2</sup>.
On March 1, 2006, Kōyaguchi was merged into the expanded city of Hashimoto.