Reginiussen made his Rosenborg debut, starting the whole game, in a 3–0 win against Sogndal IL on 12 August 2012. He then played in both legs of the UEFA Europa League Qualifiying Round against Legia Warsaw, scoring in the second leg in a 2–1 victory, and ultimately progress through to the group stage following a 3–2 win on aggregate. Since joining the club, Reginiussen became a first team regular for the side, playing in the centre–back position and formed a partnership with Stefan Strandberg. Having helped the side kept three clean sheets throughout the 2012 season, he went on to make seventeen appearances and scoring once in all competitions.