Koski Tl is a municipality of Finland.
It is located in the province of Western Finland and is part of the Finland Proper region. The municipality has a population of 2,447 (30 September 2012) and covers an area of 192.41 square kilometres (74.29 sq mi) of which 0.96 km2 (0.37 sq mi) is water. The population density is 12.78 inhabitants per square kilometre (33.1 /sq mi).
Tl in the name means the former province of Turku and Pori (Finnish: Turun ja Porin lääni). It was attached to name to distinguish Koski Tl from another municipality named Koski, Koski Hl (Hl for the former province of Tavastia, Finnish: Hämeen lääni). Koski Hl was renamed Hämeenkoski in 1995 but Koski Tl has not changed its name.
Koski Tl is unilingually Finnish.