In a battle of the last two winners of this event, Tseng fired a six-under 66 to repeat as champion. She finished at 19-under-par 269 to pick up her 13th LPGA Tour title.
Miyazato, the 2010 winner, posted a four-under 68 to take second place at minus-18.
Jiyai Shin, who briefly had a share of the lead on Sunday, carded a five-under 67 to end alone in third at 17-under-par 271.
Playing in the penultimate group, Shin birdied the 17th on the Pattaya Old Course at Siam Country Club to join Tseng in the lead. Shin could only par the par-five closing hole to end at minus-17.
Miyazato, the third-round leader, rebounded from an up-and-down front nine with four birdies on the back nine. Three of them came in the final four holes as she nearly forced a playoff.
Tseng showed her mettle down the stretch. After a pair of dropped shots early on the back nine, the world No. 1 matched Miyazato's birdies on the final two holes to claim the title.
"I had a one-shot lead and I was thinking maybe make par and I win, but after I see Ai hit it so close, I kind of thinking that I have to make birdie to win it," Tseng stated of the 18th.
"I didn't know my ball was that close. After I saw that, I was really happy. That tap-in putt was great. It feels really good to win here again."
Tseng trailed by one to start the round, but quickly erased that deficit at the par-five opening hole. Tseng made eagle, while Miyazato converted a birdie try and the two were tied at 15-under.
After a par on the second. Tseng birdied the third to take the outright lead. Back-to-back birdies from the sixth gave her a four-stroke cushion.
Miyazato tripped to bogeys on five and six, but recovered those shots with birdies on Nos. 7 and 8.
Tseng stumbled to a bogey at the 10th and was just one clear of Shin, who made a run with three birdies in a four-hole span from the seventh. Tseng pushed her lead back to two with a birdie on the 12th, but she gave that shot right back with a bogey on 13.
The 23-year-old Tseng parred three in a row from the 14th and wasn't being pressured at that point.
Shin parred six in a row from the 11th. She joined Tseng at 17-under with a birdie at 17, but parred the last to end in third.
Miyazato had dropped a shot on the ninth, but got that stroke back with a birdie at 10. She was two back at that point and remained there as she parred four in a row from No. 11. Miyazato got within one with a birdie on the 15th.
Tseng closed the event in style as she birdied the final two holes, including a tap-in at the last.
Miyazato also birdied the last two holes to end one back.
"I felt like maybe if I had some patience, there might be another chance towards the end. Then I made birdie on No. 10 and I felt like it was a re- start," said Miyazato.
"So I told myself, 'All right, I'm going to start right now.' I had two birdie chances after that. I didn't make them and maybe those two putts cost me the tournament. But overall I played really well and I'm very happy that I could finish strong today."
NOTES: Tseng earned $225,000 for the win...She needs just six more points to qualify for the Hall of Fame, but can't be inducted until after her 10th year of LPGA service...Karrie Webb was in the mix through six holes, but carded a quadruple-bogey on the par-five seventh to fall out of contention...Morgan Pressel had a hole-in-one on the eighth on Sunday after Webb and Paula Creamer had aces on No. 12 earlier in the week...The LPGA Tour heads to Singapore next week for the HSBC Women's Champions, where Webb won by two strokes last year.