Second seed Kamiji, who won singles bronze in Rio, defeated her top-seeded opponent in 2 hours, 18 minutes to better her runner-up finishes at the tournament in 2014 and 2015.
"I was really pleased the moment the match finished. I was in good form this past week," said Kamiji, who now has three of the four Grand Slam crowns after winning the French and U.S. Opens in 2014.
"Given the way I've been playing, I was wondering when I'd be able to win (this) title if I didn't win it this time. I want to become a player who can keep winning.”
Kamiji also made up for her disappointment from a day earlier when, as second seed, she missed out on her fourth-consecutive doubles title in Melbourne. She and Dutch partner, Diede de Groot, who was playing in her first Grand Slam, lost 6-3, 6-2 to the top seeded Dutch pairing of Griffioen and Aniek van Koot.