Koshiji, Niigata

 was a town located in Santō District, Niigata Prefecture, Japan.
As of 2003, the town had an estimated population of 14,160 and a density of 242.30 persons per km². The total area was 58.44 km².
On April 1, 2005, Koshiji, along with the town of Oguni (from Kariwa District), the village of Yamakoshi (from Koshi District), the town of Nakanoshima (from Minamikanbara District), and the town of Mishima (also from Santō District), was merged into the expanded city of Nagaoka.