Creamer, who turned 19 earlier this month, finished at 19-under 197, three strokes ahead of Miyazato, who shot a 69.
'I feel great,' said Creamer, who carded four birdies in her third straight bogey-free round. 'I knew I had to play good golf coming down the stretch. I knew if I just stayed patient and kept giving myself birdie looks everything would be OK.'
Miyazato, who turned 20 in June, won five titles last year when she made her debut on the Japan tour. She played in four overseas events during a two-month absence from the Japan this year, including the U.S. Women's Open and the Women's British Open.
'The biggest difference between Creamer and I today was the putting,' Miyazato said. 'I feel there's still a big gap between myself and the world's top players.'
Creamer put together consecutive birdies on the par-3 No. 8 when her tee shot landed one foot from the hole. Her fourth birdie of the day came on No. 16 when her chip shot from the greenside rough landed inches from the cup.
Creamer was making her first appearance in a Japanese LPGA tour event after winning twice on the LPGA Tour this year. She became the second-youngest first-time winner in tour history - and the youngest winner of a full event - when she captured the Sybase Classic by one stroke in May. She also won the Evian Masters in July.
Despite graduating from high school only three months ago, Creamer is second on the LPGA money list behind Annika Sorenstam.
Shiho Oyama, playing in the same group as Creamer and Miyazato, also fired a 69 to finish third at 15-under 201. Defending champion Rui Kitada was tied for 10th at 9-under 207.