Born in Vantaa, Koskinen started to play football in the youth sector of Vantaan Jalkapalloseura (VJS),[2] before joining the FC Honka youth system in 2019. He went on to make his senior debut with the club's reserve team in the third-tier league Kakkonen in 2020. Later he made his Veikkausliiga debut with the Honka first team in 2021.[3]
Lahti
After the 2023 season, Honka was suddenly declared for bankruptcy, and Koskinen was released. Soon after, it was announced that Koskinen had signed with fellow Veikkausliiga side FC Lahti on a two-year deal, starting in 2024.[4] He debuted with his new club on 27 January 2024, in a Finnish League Cup match against Inter Turku, scoring a goal in a 3–2 loss.[5]