The former top-ranked Dane has fallen to No. 11 but is showing signs of returning to her best after also winning in Seoul in September for her first title of the year.
Wozniacki dominated early against Stosur but then looked to be in trouble after the Australian leveled the match and then broke to love in the first game of the third set. But Wozniacki fought back to level at 3-all, and broke again in the last game when Stosur sent a backhand wide on Wozniacki's second match point.
"At the right times I managed to win the right points," Wozniacki said. "I tried to stay aggressive and stay close to the line and just dictate. But she played well and I knew it wasn't going to be easy."
Stosur felt she was unlucky at times in the match.
"I certainly put myself in too much of a hole to be allowed to get out of it," she said, "I feel like I went for the right things and missed by a couple of inches."
Stosur was playing in her second final this season after also being a runner-up in Doha, and is 3-12 in WTA finals.
Wozniacki has not qualified for the season-ending WTA Championships in Istanbul, but can return to the top 10 in the rankings if she wins the Tournament of the Champions, which starts Oct. 30 in Sofia, Bulgaria.
"I definitely think I played some great tennis up there," Wozniacki said when asked if she was showing the same form as when she was No. 1. "And I'm pleased about the way I've been playing at this tournament."
The hard-serving Bellucci, who hit 13 aces against Seppi, earned the only break of the first set and then served for the match twice in the second set at 5-4 and 6-5, but was broken on both occasions. The 25th-ranked Seppi, who had not dropped a set on the way to his third final this season, then won four consecutive games in the decider.
"Thomaz started well and was serving very hard and I experienced problems to return," Seppi said. "But I tried to stay close and put pressure on him. He played two bad games when he was serving for the match and it added me some confidence and helped me to win the tiebreaker and the match."
The Italian is enjoying the best season in his career with a 37-25 match record. He also won a title in Belgrade, Serbia, in May and is now 3-3 ATP tour finals.