Koivun shot 5-under-par 66 to stave off hard-charging Scottie Scheffler, who closed with a 63 at TPC Twin Cities.
Less than two months ago, Koivun was in college helping Auburn to its second NCAA championship in a three-year stretch. He demonstrated more poise while playing the final two rounds of his third tournament since turning pro without carding a bogey.
"Definitely had some nerves coming down on 18," Koivun said.
"All throughout the crowd I kept hearing, 'Scottie's coming, Scottie's coming.' I just tried to tone that out and keep playing the golf I knew I could."
Denny McCarthy (66), Brian Harman (65), Chandler Phillips (67) and Japan's Hideki Matsuyama (64) tied for third place at 20 under.
World No.1 Scheffler birdied three consecutive holes to begin the back nine as part of his bogey-free round. He birdied No.16 and 18, leaving him trailing by two strokes.
Koivun then birdied No.16, extending the margin to three strokes with two holes to play.
Koivun, a three-time Player of the Year in the Southeastern Conference college system, surged into the lead across the final nine holes on Saturday (Sunday AEST) and then went right to work on Sunday, with his approach shot on the first hole spinning back to within 2 feet of the cup for an easy birdie putt. A birdie on No.4 pushed his lead to five strokes, with Scheffler among five golfers tied for second place at the time.
"I just love this game. I love golf, it's everything I do," Koivun said.
"I have a tough time getting away from it. Like I said, it's been love at first sight."
Jake Knapp shot an early 62 to rise quickly on the leaderboard and finish at 16 under, tying for 13th.
Defending champion Kurt Kitayama (68) finished tied for 24th at 13 under.
No Australians made the cut.
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