Romania's Halep took the season-ending event 2-6, 6-2, 6-2, improving to 6-0 in finals this year.
It was her second finals victory over the Australian in 2013 -- she also downed Stosur last month for the Kremlin Cup championship.
Halep turned the match around in the second set with three breaks of her fourth-seeded opponent, adding two more in the final set.
Stosur fell to 2-2 in title matches this year after logging wins in Carlsbad and Osaka.
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