The 173 kilometre stage from Gjøvik to Hønefoss had a single big climb halfway through and eight riders broke clear of the bunch.
IAM Cycling took the responsibility to conduct the pace of the pack and the escapees were swept up before a demanding undulating finale.
When the counterattacks began Boasson Hagen's Team Sky went to the front and picked up the pace but Lars-Petter Nordhaug (Blanco) launched a fierce attack on the final climb and moved away with 12 kilometers to go.
He was chased by Jesper Hansen (Team Cult) and Fredrik Ludvigsson to form a front trio but Team Sky brought the escapees back and Katusha's Kristoff ensured it was a double celebration for Norway by claming the bunch sprint.
Kristoff beat Sondre Holste Enger (Plussbank) into second position with Boasson Hagen third for his third win of the week.
"Obviously I'm very satisfied," said Kristoff. "I closed this Tour of Norway with three victories and one second place, these results are beyond all expectations. That means I'm in a good shape and I'm working well in order to be at my top in the Tour de France.
"My team0mates brought me in a good position so in the end I could take the victory. It's a pity, if I wasn't dropped yesterday I could have fought for the general classification but I think we can be satisfied anyway."