Koijärvi is a former municipality of Finland in the former Häme Province, now in Tavastia Proper.

[1][2] [3] The homonymous lake Koijärvi, from which the Koijoki river starts, is known for the birds which make their nests by it.

The first mention of it was in 1600 as Quoiuull, "at Kojo" (adessive case, in modern Finnish orthography written Kojolla.

The area was mainly within the Tammela parish, originally called Porras.

Most of it, including Kojo, was transferred to Forssa in the south, while small portions of the north were given to Urjala.