Koski Tl is a municipality of Finland. Tl means former province of Turun ja Porin lääni (now part of the province of Western Finland). There was another municipality named Koski (Koski Hl, Hämeen lääni; now Hämeenkoski in Southern Finland).

It is located in Finland Proper region. The municipality has a population of 2,559 (2003) and covers an area of 192.50 km² of which 0.75 km² is water. The population density is 13.3 inhabitants per km².

The municipality is unilingually Finnish.