The 50-year-old Riegger birdied three of the final four holes -- holing a 20-footer for birdie on the par-5 18th -- for a 4-under 68 and a two-stroke victory over Cook. Riegger finished at 15-under 201.
"I didn't look at the board," Riegger said. "The only person to put pressure on you is yourself. I knew these guys were great players their whole careers. Still are. I knew they were going to come out and throw some birdies at me early. I was going to play my game and I knew if I played the way I was capable of playing, things would take care of themselves."
Riegger, a two-time winner on the Web.com Tour, is the fifth rookie to win this year on the 50-and-over tour, matching a record. He won $300,000, the biggest check in his professional career.
"This is going to go down as the best win of my career," Riegger said. "It's been a long, crazy career. I've been around the world and had a little success on the PGA Tour, European Tour, Web.com, but my game has actually gotten better the last couple years. ... I was just trying to play my game, do what I was capable of doing, play the smart percentage and that's what I did all day long."
Cook closed with a 65.
"For him to come out and not have that much experience, to do what he did down the stretch is very commendable," Cook said about Riegger.
Couples, from Seattle, was third at 11 under after a 66. He birdied the last three holes and four of the final five.
"Today it was a nice finish," Couples said. "In the middle, I had a couple (missed) putts and that was about as good as I could have shot."