Nordqvist, the LPGA Championship and LPGA Tour Championship winner in 2009, led wire-to-wire. The 26-year-old Swede closed with a 4-under 68 to beat defending champion Park by two strokes.
''I'm speechless to be honest,'' Nordqvist said. ''It's been a couple years since I won. I've been working very hard. Had my ups and downs. Just couldn't be happier to be here. It was such a hard push on the back nine.''
Nordqvist finished at 15-under 273 on the Pattaya Old Course. Park, making her first start of the year, had a bogey-free 66. The South Korean player won six times last year.
''I feel good,'' Park said. ''Today, my ball-striking wasn't as good as yesterday. I putted better than yesterday. So I feel like my putter is coming back. I played one shot better than last year and didn't win. Still a very good result. Tells me I improved a little.''
Scotland's Catriona Matthew was third at 11 under after a 65, and Michelle Wie was 10 under after a 69.
''Inbee kept making birdies and Michelle was playing great,'' said Nordqvist, four strokes ahead of Park and Wie entering the round. ''I couldn't really breathe until the last putt, so obviously I could just let go on the last putt.''
Wie cut the lead to one with a three-stroke swing on the par-4 fifth hole. Nordqvist had a double bogey on the hole and Wie made a birdie.
Nordqvist rebounded with a 12-foot birdie putt on No. 7 to extend her lead two shots. She birdied five of the first seven holes on the back nine to open a three-stroke lead and closed with a bogey on the par-5 18th.
''I just really had to stay strong,'' Nordqvist said.
Projected to jump to 14th in the world ranking, Nordqvist changed equipment and started working with instructor Jorje Parada during the offseason after considering leaving the tour.
''I surround myself with great people and friends and family,'' Nordqvist said. ''I have a great coach that really inspired me the last couple months, couple weeks, just to believe in myself and keep going no matter what. It was a grind this offseason, but sitting here with the trophy it was well worth it.''
Yani Tseng, the winner in 2011 and 2012, had a 66 to tie for fifth at 9 under with Stacy Lewis (66), Lexi Thompson (68), Gerina Piller (66) and Julieta Granada (69). Lewis has 13 straight top-10 finishes, three off the LPGA Tour record set by Karrie Webb in 2000.