The Swede got down well to keep out Shaun Williams’ 78th-minute spot-kick in a tight game at the Keepmoat Stadium as Doncaster picked up first point of the campaign.
Tommy Rowe looked to have given Doncaster the lead after 19 minutes when he turned in Ben Close’s centre at the second attempt, only for the goal to be ruled out for a foul in the build-up.
The game kicked into life in the second half, with Rovers keeper Dahlberg producing a pair of excellent reaction stops to keep out efforts from Michael Jacobs and John Marquis.
At the other end Charlie Seaman almost caught out Gavin Bazunu with a powerful drive, but the Pompey keeper recovered at the second attempt.
Portsmouth were given a golden chance to go in front when Ro-Shaun Williams bundled over Gassan Ahadme in the box, but Williams’ effort was saved.
Branden Horton’s volley almost looped over Bazuna late in the game as Doncaster pushed hardest for a winner, but both sides had to settle for a point.