At the start of the 2018 season, Reginiussen came on as a second half substitute, and helped them beat Lillestrøm 1–0 to win Mesterfinalen for the second time in a row. He then helped the side kept four clean sheets between 8 April 2018 and 29 April 2018; during in which, he scored in a 2–0 win against IK Start. Since the start of the 2018 season, Reginiussen continued to regain his first team place, playing in the centre–back position, forming a partnership with Even Hovland. A month later on 30 May 2018, he scored the winning penalty for Rosenborg, as they beat Odds 7–2 in the shootout after the game went to extra time, with a 2–2 scoreline. Two months later on 11 June 2018, he scored his second goal of the season, in a 2–1 loss against Tromsø. This was followed by helping the club keep two clean sheets in the next two matches against Vålerenga and Sandefjord. Reginiussen helped the side qualify for the UEFA Europa League Group Stage after beating KF Shkëndija 5–1 on aggregate; in which he scored in the second leg. In a match against IK Start on 11 November 2018, Reginiussen started the match and helped them to keep a clean sheet, as they won 1–0, a victory that saw the club become Champions for the fourth time in a row. In a follow–up match against Bodø/Glimt, he set up a goal for Yann-Erik de Lanlay, as they drew 1–1. A month later on 2 December 2018, Reginiussen started the match, as he helped the side beat Strømsgodset win 4–1 win to win the tournament, his third time of his career. Reginiussen scored his fourth goal of the season, in a 1–1 draw against RB Leipzig on 13 December 2018. At the end of the 2018 season, he went on to make forty–seven appearances and scoring four times in all competitions.