Ahvenkoski is a historical site in the Kymenlaakso region, Finland, located by the westernmost branch of the river Kymijoki. It consists of the Vähä-Ahvenkoski village in Loviisa and the Ahvenkoski village in Pyhtää. The site is especially known of its rich military history. Between 1743 and 1809, Ahvenkoski was a border crossing of Sweden and the Russian Empire. Ahvenkoski was originally part of the Pyhtää municipality. As the border divided the village in two, the Swedish side became known as Ruotsinpyhtää, the Swedish Pyhtää, which was annexed to the town of Loviisa in 2010.