The 21-year-old, who has been capped by Sweden at senior level, is struggling for action at Vicarage Road.
Dahlberg was consequently loaned to Dutch side Emmen in January and he has now been sent out once again.
Swedish top-flight side BK Hacken have agreed a deal with Watford for the goalkeeper and he will spend the whole of the 2020-21 season with them.
Dahlberg’s departure means Watford have lost two goalkeepers this summer, with 39-year-old Brazilian Heurelho Gomes announcing he was leaving last month.
The club are hopeful Ben Foster, who signed a new two-year deal with them in June, will stay put and be the team’s No1 in the Championship next season.
Daniel Bachmann, meanwhile, is likely to act as the team’s back-up goalkeeper and compete with Foster to be No1.