After the round, Jutanugarn was consoled by younger sister Ariya, who won her first major championship previous year.
Both players shot 66 to tie on nine under par, and a bogey five at the first play-off hole - the 18th - was good enough for the 30-year-old to add another major to the 2009 LPGA Championship.
The 2009 LPGA Championship victor, who battled glandular fever (mononucleosis) this summer, outlasted third-year-pro Altomare under borderline-unplayable conditions similar to those on Thursday that forced the LPGA to reduce the tourney to 54 holes.
Moriya went 3 under par on Saturday to improve to 9 under, three strokes clear of first-round leader Park Sung-hyun, one of Saturday's late starters who had completed 11 holes as of 9.30pm Thailand time, and former world No.1 Lydia Ko.
Second-round leader Moriya Jutanugarn of Thailand and two-time major victor Lydia Ko of New Zealand fell out of the playoff by dropping shots on the 18th. The two finalists shot a 204. She also birdied the 18th from three feet to sneak into second place.
Two holes behind, Jutanugarn was struggling at the 13 hole. The condition sidelined her throughout the summer, resulting in her coming up one event short of the required number events needed to qualify for the European Solheim Cup team.
When she combined for a birdie at the 14th and a chip in for eagle at the 15, she found herself tied at the top at nine-under par with Nordqvist, Altomare and Jutanugarn.
"I played great today and hopefully I can keep it going", said the 33 year-old.
This time, she is hoping she can hold onto the lead and finally claim her first professional title.
Not content with settling for a playoff, Ko grabbed the fairway wood, and while she avoided the water hazard, her approach landed in the rough to the left of the green. She could well have joined the playoff if not for the double bogey at the par-four 10th. It was during U.S. Open, and it wasn't - I wasn't feeling very well that week, but still battled through; eventually found out that I had mono.
"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".
She claimed her third victory on tour, and first in seven years, at the Thornberry Classic at Wisconsin in July, when she admitted she nearly gave the game in 2015 and 2016 following seasons of struggle.