'This has been a great week,' said Sorenstam, who won for the eighth time this season. 'I'm very proud of this year.'
Sorenstam carded a 3-under 69 to finish regulation alongside Kerr at 13-under-par 275. On the first playoff hole, Kerr took herself out of the mix with an unfortunate approach shot that found the water while Sorenstam missed the green. Sorenstam chipped to 7 feet and calmly two-putted for a bogey and the win.
'The playoff hole, it wasn't as pretty as all the previous holes we played,' said Sorenstam, who also won this event in a playoff in 1997.
Karrie Webb and Carin Koch shared third place at 8-under-par 280. Jennifer Rosales, who played alongside Sorenstam in the final round, struggled to a 74 to finish six shots off the pace at 7-under-par 281.
Sorenstam had a shaky third round, but salvaged her Saturday with three late birdies that helped her carry a one-shot lead into the final round. The top player in the game looked to be in trouble again in the early goings on Sunday with a bogey at the par-3 fifth.
The 34-year-old recovered quickly and knocked her approach inside 16 feet for a birdie at the par-4 sixth. Sorenstam then hit her tee shot to 30 feet and drained the putt for another birdie.
At the par-5 ninth, Sorenstam's second shot stopped short of the green but she chipped to 6 feet and sank the putt for a birdie to match Kerr at 12- under.
Sorenstam was coasting on the back side as Kerr took the outright lead again. Sorenstam then played a 9-iron within 6 feet of the hole for a birdie at the par-4 16th to match Kerr again. She then parred her two remaining holes before improving her playoff record to 12-5.
'A win is a win,' said Sorenstam, who earned her 56th career victory on the LPGA Tour. 'By the end of the day, the trophy is in my hand. I'm happy.'
Kerr jumped out of the gate on Sunday with a birdie at the par-4 second after her second shot stopped within 12 feet of the hole. She then birdied the par-5 third and added a birdie at the par-5 ninth to make the turn at 12 under par.
The 27-year-old shared the lead with Sorenstam throughout the final round but briefly took sole possession of first with a birdie at the par-5 15th that lifted her to 13-under. Kerr was soon joined again by Sorenstam and in the end was unable to hold off the Swede.
'I played my heart out and I'm a little disappointed, but not a whole lot, because I did all I could do and I was faced with a really, really tough shot in the playoff,' said Kerr, who shot a 68 on Sunday. 'It wasn't meant to be.'
Meg Mallon, who won this event in 2003, tied for seventh place with Rachel Teske at 4-under-par 284. Juli Inkster and Jeong Jang shared ninth place at 1-under-par 287.