Hjertø-Dahl, a 20-year-old Norway Under-21 international, was in East Yorkshire this week as the two clubs worked to finalise a deal worth around £8.5m to £10m, with the Norwegian midfielder set to train with his new team-mates at Cottingham ahead of Hull's trip to Frankfurt this weekend.
Hull City moved quickly to beat competition from Rangers, Cardiff City and RB Salzburg for his signature, with Tromsø's asking price proving too rich for the Scottish club, who saw an earlier £5.4m bid rejected, reports the Hull Daily Mail.
Tromsø sports editor Rune Robertsen described Hjertø-Dahl as possessing a "considerable physical size in combination with good feet" who "can also shoot well from distance," adding that the midfielder – who has drawn comparisons to Jude Bellingham from a former coach – has increasingly played in a more advanced role for Tromsø after starting his senior career as a defensive midfielder.
"I think they will be surprised by the speed of the ball and Jens' technical ability," Robertsen said of Hull supporters' first impressions, while cautioning that the gap between the Eliteserien and the Premier League "is far apart" and it remains to be seen how quickly he can adapt.
In the other direction, 23-year-old goalkeeper Thimothee Lo-Tutala has dropped down to League One, agreeing a season-long loan at Colchester United. French-born but raised in Essex, he progressed through academy football at West Ham, Tottenham and Brentford before settling into senior football with Doncaster Rovers, where last season's loan produced 38 appearances.
The move represents Colchester's sixth addition of the summer, with Lo-Tutala set to battle Tom Smith – who put pen to paper on a new deal last week – for the goalkeeping jersey.
Both pieces of business reflect Hull's broader approach to squad-building since promotion back to the Premier League, with the club keen to add proven top-flight quality in key areas while continuing to develop younger players through loan spells elsewhere.
Hjertø-Dahl's arrival would represent one of the club's more significant outlays of the summer, underlining the priority placed on strengthening the middle of the park as Hull count down to their first season back in the top flight.