Three strokes behind leader Tseng entering the round, the third-ranked Pettersen closed with a 5-under 67 for a two-stroke victory over second-ranked Stacy Lewis.
Pettersen finished at 20-under 268 for her 12th LPGA victory. The 32-year-old Norwegian also won the LPGA Lotte in Hawaii in April and won a Ladies European Tour event in March in China. She won the 2011 tournament at Pumpkin Ridge.
''I felt like this was a good bounce-back from last week when I kind of screwed it up on the last day,'' said Pettersen, who finished eight strokes behind winner Lydia Ko in the Canadian Women's Open after beginning the final round a shot back.
''I tried to give myself chances. Once I got going I felt like I made a lot of clutch putts. I guess that's what made me win today. The putter was a good friend of mine.''
Tseng followed her third-round 63 with a 78 to tie for ninth at 12 under. The Taiwanese star, a 15-time winner on the tour who had a 109-week run at No. 1 in the world ranking, is winless in 37 events since the Kia Classic in March 2012.
Lewis, a three-time winner this year, shot a 68.
''It was a little frustrating. I played really good, though, so I can't be upset,'' Lewis said. ''A lot of those putts I hit exactly where I wanted to and they just didn't go in. A bogey-free tournament I've never done before, so I keep checking things off the list.''
The tournament returned to Columbia Edgewater after four years at Pumpkin Ridge. Columbia Edgewater also was the tournament site from 1990-2008.
For Pettersen, the Safeway victory capped an emotional three-week period. It started with Solheim Cup, where Pettersen helped Europe rout the United States in Parker, Colo.
''Seems like Portland is a good stop for me,'' Pettersen said. ''Seems like I always play well here. This was a good finish to a solid three-week stretch for me.''
A key point during Pettersen's final round was her response after hitting a poor 8-iron at the par-3 second hole that led to a double bogey.
''I felt like an amateur, to be honest. That was a disaster of a hole,'' Pettersen said. ''I decided before I went out (Sunday) just not to have too many reactions, good or bad. I knew there were birdies out there if you really get it going.''
Pettersen quickly with a birdie on the par-4 third that started a stretch where she birdied five of the next six holes and eight of 11.
Pettersen made an 18-foot birdie putt at the par-3 eighth to overtake Lewis for the lead, curled in a 5-footer for birdie on the par-5 10th and made a 25-footer from off the green on the par-4 11th.
Pettersen opened up a four-stroke lead with an 8-foot birdie putt on the par-3 13th. She ran into trouble at the 175-yard, par-3 16th, following a short tee shot with a poor chip. But she minimized the damage when her next chip rolled within 3 feet to set up a bogey-saving putt.
Pettersen said she didn't know she had a three-stroke lead on the 16th tee.
''It was a pretty bad swing. Even worse chip. But it was a good 4,'' Pettersen said.
Pettersen closed out the tournament with two pars.
She was greeted on the 18th green by Sophie Gustafson, the longtime European Solheim Cup player who announced her retirement from the LPGA on Friday.
''We have a history together and to see her on the 18th was a little bit special,'' Pettersen said. ''I never thought she would kind of stick around. When she says she is done, she's done. She is usually out of there. One thing is to get to know each other on the golf course, but she has become a good friend of mine and that's what you do for friends.''