Andrade, who had led after each of the first three rounds, was leading by one when Forsman poured in the putt on the final hole to leapfrog him and get to 14-under.
'Instinctively, when you look up, you judge the speed of the ball and think, 'OK, that looks good,'' Forsman said. 'I had a sense in my heart that it was going to be a good putt.'
Needing a birdie to force a playoff, Andrade hit his tee shot on 18 into the right fairway bunker. His ball came to rest near the lip of the hazard. His second shot clipped the lip and trickled into the fairway, from where he hit his approach shot 25 feet left of the hole.
His birdie effort came up a foot short.
'We got to the tee and that rainstorm came in,' Andrade said. 'It was howling left-to-right and it was a hard tee shot. I wasn't in sync there. These are the reasons you win tournaments and lose tournaments.'
Forsman shot a flawless 6-under-par 65 at the Waynesborough Country Club to win his first PGA Tour event in 246 starts. The victory was his fifth on tour, but his first since the 1992 Buick Open.
The 44-year-old out of Provo, Utah, earned $594,000 ' more than $400,000 better than his previous biggest paycheck on tour.
He started the week 73-68, but shot a course-record 64 in Round 3 to get within three of Andrades 54-hole lead.
Sunday, he made four birdies, no bogeys, and the eagle at the last.
Andrade (69) finished tied with Robert Allenby (65) for second place. Allenby had a 12-foot eagle at 18 to get to minus 14, but left it inches short. He also bogeyed the par-3 17th ' the same hole he aced the day before.
'To tell the truth, the only thing I was disappointed in was my shot, my club into 17,' Allenby said. 'I hit a perfect shot, but it came up short and I had no second shot.'
Allenby has three top-3 finishes - without a victory - in his last three starts on tour.
Andrade seemingly took command of the tournament Sunday with four straight birdies starting at the sixth. However, he double bogeyed the par-4 10th to open the door for others.
After birdieing the 13th to regain sole possession of the lead, Andrade made a costly bogey-6 at the par-5 15th. He then parred his remaining three holes to remain winless since the 2000 Invensys Classic at Las Vegas.
Final results from the SEI Pennsylvania Classic