The victory was the first in four seasons on the tour for Miyazato.
The 24-year-old Miyazato shot a 69 and Gustafson a 70 in the final round, tying at 14-under-par 274 and setting up the playoff.
"I was dreaming about this when I was young, so my dream has come true," Miyazato said after her win. "I went through a lot of tough times, especially the last couple years."
Miyazato won $487,000.
Meena Lee (65) and Cristie Kerr (70) shared third place at 275.