Altomare closed with a 2-under 70 to match Broch Larsen at even-par 70. Broch Larsen had a 75.
"It is an incredible feeling," Altomare said. "I feel like I am on a high."
After Altomare and Broch Larsen each parred the par-5 18th four times before Altomare broke through for her first career victory with a 4-foot birdie putt.
"The moment that ball went in, it was almost like a relief like, 'I did it' finally," Altomare said. "This is my fourth year as a professional and it is exciting, it is a relief and a confidence booster."
The 25-year-old former University of Virginia player from Shrewsbury, Massachusetts, earned $16,500. She expects to play in the next six LPGA Tour events are gaining status at the money-list reshuffle.
"This win gives me an incredible amount of confidence," Altomare said. "I know my game is ready and I know I can do it so I am just excited."
Thailand's Sherman Santiwiwatthanaphong was third at 3 over after a 70.