Purtzer closed with a 4-under 68 to match Roberts (69) at 10-under 206 on the Valencia Country Club course. The 55-year-old Purtzer earned $240,000 for his fourth career victory on the 50-and-over tour.
'I feel really fortunate, and lucky,' said Purtzer, in tears after the victory.
After the two players matched pars on their first three trips down the par-5 18th in the playoff, Purtzer finally broke through with his 17-foot birdie putt. Roberts had a chance to match, but his 12-foot birdie try just missed on the right side.
Purtzer credited fellow tour player Dave Stockton for some recent putting help.
'I have to thank Dave. He gave me a lesson and I putted better,' said Purtzer, a five-time winner on the PGA TOUR. 'I hung in there and played really well.'
Purtzer, playing in the group in front of Roberts, made a 10-foot birdie putt on No. 18 in regulation to reach 10 under. Roberts, hampered by back problems, had a chance to win in regulation, but missed a 19-foot birdie try.
'It was a long day,' Roberts said. 'We took it as far as we could go. I was just happy to get in a playoff. My back has been really bothering me.'
David Eger (69) finished a stroke back at 9 under.
Hale Irwin, the leader the first two days, closed with a 73 to drop into a fourth-place tie with Tom Jenkins (68), Joe Ozaki (67), Des Smyth (68), Jim Thorpe (71) and Andy Bean (72) at 8 under.
'I played 27 good holes and 27 not so good,' the 61-year-old Irwin said. 'I just didn't do what I needed to do. I played very poorly.'