The 20-year-old had developed into a key player for Tromsø, scoring seven goals in 14 Eliteserien appearances this season to sit joint-second in the division's scoring charts at the time of his departure, and was tracked by Leeds United, Brighton and Bournemouth earlier in the year.
Rangers had been long-term admirers and lodged an opening bid of around £5m, but withdrew from the race after being unable to match Tromsø's asking price, while Cardiff City also made a strong push before missing out. Reports in Norway put the final fee at around £8.5m, though some English outlets have suggested it could rise closer to £10m.
Hjertø-Dahl, who joined Tromsø's academy at the age of six and made his first-team debut as a 17-year-old, went on to make 105 senior appearances for his hometown club, scoring 19 goals and providing 11 assists, and helped the side finish third in the Eliteserien in both 2023 and 2025.
He becomes Hull City's sixth signing of the summer as the newly promoted Premier League club build towards their return to the top flight, which begins at home to Manchester United on 22 August.
"I'm very happy. It's a huge club and I'm really looking forward to playing here.
"I heard about the club's interest just over a week ago. From there, it's happened very quickly," Hjertø-Dahl said. He also explained what convinced him to make the move.
"I spoke to the chairman on a video call and he told me lots of good things about the club. I also spoke to Sergej and I really like his vision for me and for the team."
Reflecting on his exit from his boyhood club, the midfielder struck an emotional tone: "The last three-and-a-half years with the senior team were the greatest years of my life at Tromsø. I will miss them, but I'm ready for Hull and what's waiting here.
"I needed a challenge, and this is the biggest challenge you can get. I think this is the right step and that Hull can help me improve."