Playing career
Halttunen made his professional debut for IFK Helsingfors on 24 September 2022, in a game against KooKoo Kouvola. 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. In August 2023, he signed for the London Knights of the Ontario Hockey League. In the 2024 playoffs Halttunen led the OHL in playoff goals as the Knights won the J. Ross Robertson Cup. 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. Halttunen was named to the Memorial Cup All-Star Team.
After participating in the San Jose Sharks' training camp, Halttunen was assigned to play with San Jose's American Hockey League (AHL) affiliate, the San Jose Barracuda, to begin the 2024–25 season. He had four points in six games before being sent back to the OHL for the remainder of the season. The London Knights won their second consecutive Robertson Cup, with Halttunen recording 21 points. He scored nine goals in the championship series against the Oshawa Generals, the most for an OHL player in the playoff finals since Jonathan Cheechoo in 1999. In recognition of this, Halttunen received the Wayne Gretzky 99 Award as the most valuable player of the postseason. The Knights went on to compete at the 2025 Memorial Cup, ultimately defeating the Medicine Hat Tigers for the title.
On 23 June 2026, Halttunen was traded to the Ottawa Senators alongside forwards William Eklund and Brandon Svoboda for a 2026 first-round pick.
