In January 2017, Trueman was signed by the Castleford Tigers, joining the club on a two-year deal. He made his Super League début on 4 June 2017 against St. Helens. He scored a hat-trick in his second Castleford appearance against Wigan Warriors on 17 September. At the end of the season, Trueman was named as the Tigers Academy Players' Player of the Year. In December 2017, Trueman signed a new three-year contract with Castleford, with head coachDaryl Powell labelling him "the most promising half-back in the English game". Trueman was assigned squad number 21 for 2018. He was utilised in the fullback position for a period, although he made the majority of his appearances at halfback. He performed consistently well throughout the season, cementing his place in the first team by appearing in all but 4 of Castleford's games. Trueman's contributions were reflected in the end-of-season awards, as he was named as Castleford's Young Player of the Year and 2nd place Player of the Year. He also went on to win the Super League Young Player Of The Year 2018. In November, he signed a new four-year contract with the Tigers until the end of 2022. For the 2019 season, Trueman was given squad number 6, recognising his place in Castleford's first team. Injuries to teammates meant that he wasn't able to form a consistent partnership with any other halfback, starting alongside Jordan Rankin, Paul McShane, Peter Mata'utia, Cory Aston and Jamie Ellis at different points throughout the season. He scored his second hat-trick for Castleford against Hull F.C. on 5 September, to help the Tigers reach the play-offs. He made 31 appearances, missing only 1 game for the Tigers, and scored 8 tries. He finished as a runner-up for Castleford's Player of the Year and Players' Player of the Year awards and was once again nominated for Super League Young Player of the Year. Trueman was named Man of the Match for his performance against the Toronto Wolfpack in the opening round of the 2020 season. His developing halfback combination with new Tigers signing Danny Richardson is widely lauded as an exciting and potentially long-term partnership for both club and country. Castleford captain Michael Shenton shares this view and spoke very highly of Trueman: "He is a quiet person off the field but on the field we are trying to make him grow because he’s unequalled in talent for our team. He’s our best player."
