Reitan, who chose golf over soccer at a young age, won twice previously on the European Tour. He finished at 15-under 269.
"This feeling is just unreal," Reitan said after emerging from a packed Sunday leaderboard with a final-round two-under-par 69.
Alex Fitzpatrick, the third-round leader, finished three shots back after shooting 73.
Reitan, a PGA Tour rookie who started the round one shot behind Fitzpatrick, was even par for the day after 13 holes and one shot behind Fowler, who surged to the lead after shooting 30 on the front nine.
But Fowler, who started the day seven shots behind, couldn't sustain the momentum in a final-round 65.
Playing four groups ahead of the leaders, Fowler missed a six-foot birdie putt on 16, opening the door for Reitan, Fitzpatrick, and Hojgaard to pull into a four-way tie for the lead after all three made birdie.
Reitan took the lead for good with a two-putt birdie on 15 after Fitzpatrick's wedge got caught up in the thick rough and his chip barely made the fringe, leading to a costly par on the par 5.
Hojgaard made bogey on 16 to fall two shots back.
Fitzpatrick faltered on 17 as his chip from the rough never made the green resulting in a double bogey, taking him out of contention.
Reitan played the final three holes of the Green Mile in even par and secured the win with a par on 18.
Reitan's best previous finish was a tie for second last month at the Zurich Classic when he and playing partner Kris Ventura lost by one stroke to Fitzpatrick and his brother, Matt.
Min Woo Lee fired the equal low round of the day, a seven-under 64 featuring six birdies and an eagle on the reachable par-4 14th hole, to be the leading Australian in a tie for 14th at seven under.
Adam Scott closed with a 69 to share 24th at four under in an equally encouraging finish ahead of this week's PGA Championship, men's golf's second major of the year.
With AAP
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