In driving rain, Nordqvist and Altomare returned to a soaked hole 18 for the playoff, which the Swede closed out with a bogey. Two holes later, play was stopped.
"I am from Sweden and I'm freezing", said Nordqvist, who was confined to bed for two weeks in July by a bout of mononucleosis.
"It's really big", the 26-year-old MA native said.
"I'm looking forward to finishing off the year and maybe get another W. The girls are so good out here; it's just fun to be able to compete again and know I've got the right stuff". It was a 54-hole event after weather-affected play on Thursday was scrapped.
The American would birdie the 17th to join Nordqvist, Lydia Ko, Moriya Jutanugarn and Katherine Kirk in the lead but one by one Kirk, Jutanugarn and Ko would drop shots over the closing holes to cost themselves a chance at victory.
Nordqvist's victory ensured 10 different major winners in the last two seasons.
For the 30-year-old, this is her second LPGA win this season and the eighth of her career.
She was caught by Anna Nordqvist.
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The former world number one will be a good chance to continue that strong form when she tees up at the New Zealand Women's Open, starting on September 28 at Auckland's Windross Farm Golf Course.
Altomare, who played college golf at Virginia, posted six birdies to earn her spot in the playoff. A chip-in birdie at 16 was the highlight of her round.
It was soon a five-way tie for the lead when Kirk birdied the par-5 15th and Jutanugarn's birdie putt rolled round the rim. "Just probably not really my day", Jutanugarn said.
Sophia Schubert, the US Women's Amateur champion, made her mark playing alongside the professionals with an impressive opening 68, two better than defending champion Chun In-Gee and seven ahead of world No.1 Ryu So Yeon.
American Paula Creamer was forced to withdraw on Friday with a sprained tendon in her left wrist.
Today she overcame a rocky start to her final round with a blistering stretch of holes from the 6th.
Jutanugarn's first-round 65 put the young Thai player in prime position to claim her first LPGA title. Her wins at Evian came before it was upgraded in 2013.
After puddles on the 18th green had been temporarily cleared by the greenkeepers for the sudden-death play-off, both players were off the green in three shots at the par four.