Leading striker Alfred Finnbogason – who was on loan at the Swedish club from Belgian side Lokeren – has joined Heerenveen in Holland for £450,000.
The Icelandic international confirmed he has put his name to a three-year contract with the option for another 12 months.
Finnbogason said on his Twitter page: "Hard to say goodbye to HIF, will always have a place in my heart.
"But am happy to be a Heerenveen player now."
Throughout his five months on loan in Sweden, Helsingborgs had attempted to agree a fee with Lokeren for a permanent transfer.
However, the Belgians ran out of patience, and this opened the door for Heerenveen to make their move.
At the weekend, Celtic coach Garry Parker watched Finnbogason score twice in the 7-2 win over Kalmar, which took his tally to eight in his last nine league games.
He was also on the mark when the Swedes defeated Slask Wroclaw in Poland in the previous round of their Champions League qualifiers.
Helsingborgs have now stepped up their bid to bring in Kermit Erasmus from South African club SuperSport.
Meanwhile, Mark Wilson has won a contract with Bristol City until January.
The 28-year-old full-back left Celtic at the end of last season after six years at Parkhead.
A chronic knee problem curtailed his appearances with the Hoops, but Wilson won four SPL titles, two Scottish Cups and two League Cups and proved excellent value for the £500,000 paid to Dundee United in January 2006.
A delighted City boss Derek McInnes said: "Mark had nothing to prove to me in terms of his ability and he has proved his fitness with us throughout pre-season."