Halep easily beat Williams just four days earlier during the group stage of the tournament, but the American turned that around with an aggressive game plan.
Williams won 11 of the last 12 games in the match to join Martina Navratilova and Steffi Graf as the only players to have won five titles in the season-ending championships.
The 33-year-old pulled out of her previous two tournaments with illness and a knee injury, yet still claimed the $2 million first prize, claimed the Billie-Jean King Trophy and also had a breed of orchid named after her in recognition.
“I just started training, because I had such a bad knee in Beijing I didn't know if I'd be able to play here, and now I have won the Billie-Jean King trophy — I am so excited,” Williams said.
The match was the most one-sided championship match in the WTA Finals since Kim Clijsters beat Amelie Mauresmo 6-2, 6-0 in 2003.
Williams changed her approach from the group-stage contest, attacking Halep's second serve with fierce ground strokes, and regularly rushing the net to cut the points short and prevent her opponent from playing her preferred style.
“I had to play more Serena-style tennis and just do what I do best,” Williams said.
“I was expecting a much better player than I was in the earlier round. Being ready for that really was able to help me out.”
Andy Murray wins Valenica Open
Andy Murray saved five match points before fighting back to beat Tommy Robredo 3-6, 7-6 (7), 7-6 (8) in a grueling Valencia Open final on Sunday.
Last month, Murray also saved five match points against the Spaniard on his way to winning the final in Shenzhen, China. This time, he had to do it in front of Robredo's home fans, who ended up applauding the effort made by both players on Valencia's indoor hard court.
Robredo came from a break down in the second set to force a tiebreaker that could have earned him his first win against Murray since 2007. Murray looked flustered, slamming his racket down twice, but he managed to save two match points before leveling at a set each.
The Briton then saved three more match points in the final tiebreaker before hitting a decisive backhand to win in 3 hours, 20 minutes, making it the longest final on the ATP circuit this year.
It was Murray's 31st career title and second in Valencia after winning in 2009.
The two-time grand slam winner will rise to No. 5 in the rankings with his third title of the year, after he also beat David Ferrer last weekend to win in Vienna.
Roger Federer claims Swiss Indoors title
Top-seeded Roger Federer won a sixth Swiss Indoors title, comfortably defeating David Goffin of Belgium in straight sets to end a run of two successive final defeats at his hometown tournament.
Federer took just 52 minutes to win 6-2, 6-2, delighting the partisan crowd as he inflicted what was only Goffin's third defeat in 46 matches.
“I didn't expect this kind of result but clearly I'll take it, it's great,” Federer said. “It's indoors tennis, sometimes you just blow a guy out of the court and I'm happy it was me today doing that.”
The first set lasted 23 minutes as Federer dropped just one point in the final three games.
The second-ranked Federer also moved a step closer to clinching the year-end No. 1 spot for a record-equaling sixth time as he moved to less than 500 points behind Novak Djokovic, with his fifth victory of the season