Kosaza (小佐々町 Kosaza-chō) was a town located in Kitamatsuura District, Nagasaki, Japan.
On March 1, 2006 Kosaza, along with the town of Uku, also from Kitamatsuura District, was merged into the expanded city of Sasebo.
As of 2003, the town had an estimated population of 7,063 and a density of 236.06 persons per km². The total area was 29.92 km².