In the 2019 season, Reginiussen continued to regain his first team place, playing in the centre–back position and forming a partnership with Hovland. He scored his first goal of the season, in a 3–1 loss against Stabæk on 14 April 2019. His second goal then came on 19 May 2019, in a 3–2 win against Mjøndalen IF. A month later, he captained the side for the first time since 2016, as Rosenborg lost 1–0 against Tromsø. Three days later, Reguiniussen scored an equalising goal in a 1–1 draw against Aalesund in the Round of 16 of the Norwegian Cup; which the club lost on penalty shoot–out. It was announced on 1 August 2019 that Reginiussen signed a contract with Rosenborg, keeping him until 2020. He then helped the club beat Linfield, BATE and Maribor to progress through on three UEFA Champions League rounds. However, Rosenborg was eliminated in the UEFA Champions League qualifying round after losing 3–1 on aggregate. He scored his third goal of the season, in a 3–2 win against Bodø/Glimt on 10 October 2019. However, throughout 2019 season, the club were unsuccessful to defending their league title after surrendering the title to Molde. Despite suffering from injuries later in the 2019 season, Reginiussen went on to make thirty–eight appearances and scoring four times in all competitions.