Niku made his
Liiga debut playing with
JYP Jyväskylä during the
2014–15 Liiga season. Following the
2016–17 season with JYP, having played his first full season in the Liiga in posting 27 points in 59 games, Niku agreed to a three-year, entry-level contract with the Winnipeg Jets on May 15, 2017. He played most of the
2017–18 season with the
Manitoba Moose, the Jets'
American Hockey League (AHL) affiliate. He was called up by the Jets on April 1, 2018, and scored his first goal in his NHL debut in a Jets' win over the
Montreal Canadiens on April 3. At conclusion of the
2017–18 regular season, Niku was voted as the
AHL's top defenseman, as well as earning
AHL First All-Star team and the
AHL All-Rookie honours. On 21 September 2021, Niku was bought out of his contract with the Jets. Three days later, he signed a one-year contract with the
Montreal Canadiens. A week later, in his first pre-season game with the team, Niku sustained a concussion after being hit from behind by
Ottawa Senators player
Josh Norris and was ruled out indefinitely. As a free agent from the Canadiens, Niku opted to return to Finland agreeing to re-join original club, JYP Jyväskylä of the Liiga, on a one-year contract for the
2022–23 season on 29 July 2022. For the 2024-25 season, Niku signed a one-year contract with the Swiss club
EHC Kloten of the
NL. On April 30, 2025, Niku signed a two-year contract with the
Lausanne HC, remaining in the NL. ==International play==