Playing career
On 7 February 2024, while playing with Mikkelin Jukurit of the Liiga, Helenius became the second youngest player in Liiga history to record a hat trick, after Tuomo Ruutu, and fifth to do so before the age of 18. He finished the season with 14 goals and 36 points in 51 games. Following the season, he was selected 14th overall in the 2024 NHL entry draft by the Buffalo Sabres, with whom he signed a 3-year entry-level contract on 8 July.
Helenius spent the entire 2024–25 season playing with the Rochester Americans in the AHL. During the 2025–26 season Helenius recorded 30 points (9-G, 21-A) in 34 games with the Rochester Americans en route to being named to the 2026 AHL All-star Classic. On 15 January 2026 Helenius was called up to the Buffalo Sabres later making his NHL debut on 19 January. On 20 January, in his second career NHL game, Helenius scored his first NHL goal, added two assists and was named the first star in a 5–3 win over the Nashville Predators.
Helenius made his playoff debut as a black ace during the 2026 playoffs, scoring his first NHL playoff goal in his second playoff game, a 6–3 loss to the Montreal Canadiens on May 14.
Finland
