[2] Halttunen made his professional debut for IFK Helsingfors on 24 September 2022, in a game against KooKoo Kouvola.
[3][4] Following his selection by the Sharks, as the first Finnish prospect chosen in the 2023 NHL entry draft, Halttunen was signed to a three-year, entry-level contract with San Jose on 12 July 2023.
[7] As OHL champions, the Knights played in the 2024 Memorial Cup in Saginaw, where they reached the championship game but were defeated 4–3 by the host Saginaw Spirit, in a rematch of the teams' semi-final series during the OHL playoffs.
[9] Halttunen represented Finland at the 2022 IIHF World U18 Championships where he recorded three goals and two assists in six games and won a bronze medal.
Halttunen also won a silver medal at the 2025 World Junior Ice Hockey Championships while representing Finland.