HJK Casper Terho made his professional debut for
Helsingin Jalkapalloklubi on the 26 August 2020, coming on as a substitute in the
Veikkausliiga 2–0 home win against
Seinäjoen Jalkapallokerho. Along with other youngsters such as
Kai Meriluoto, he played a few other games by the end of season, where his team ended up winning the
2020 championship. Being already viewed as one of the brightest hopes from the
Finnish champions academy, he scored his first goal in his
2021 season debuts, during the 4–2 home
championship win against
FC Honka on the 24 April 2021. By the end of May, he became a regular starter with
HJK, scoring another goal against
Ilves, while still aged only 17.
Royale Union Saint-Gilloise On 29 September 2022, Belgian side
Union SG announced that they had reached an agreement with HJK about his transfer. He joined the Belgian side on 1 January 2023 on a deal until 30 June 2026, for an undisclosed fee, with HJK securing a sell-on clause. He officially made his debut for the club on 8 January against
Anderlecht, as a 77th-minute substitute. Terho scored his first goal for the club on 20 April 2023, in the first leg of the
2022–23 UEFA Europa League quarter finals against
Bayer Leverkusen. Despite his goal, his team lost the game 4–1 (5–2 on aggregate), and were eventually knocked out of the competition. On 23 April 2023, Terho scored his first goals in the
Jupiler Pro League, a brace in a 4–2 away win against
Kortrijk. In 2024, Terho and Union SG won the
Belgian Cup and the
Belgian Super Cup.
SC Paderborn (loan) On 1 January 2025, it was announced that Terho was loaned out to
2. Bundesliga club
SC Paderborn 07 for the remainder of the season with a purchase option.
OH Leuven On 3 July 2025, Terho signed with Belgian Pro League club
OH Leuven on a deal until 2028 for an undisclosed fee.
Sparta Rotterdam (loan) On 2 February 2026, it was announced that Terho was loaned out to
Eredivisie club
Sparta Rotterdam for the remainder of the season with a purchase option. == International career ==