MORE: Halep laments nerves after Williams thrashing
Halep had handed Williams a heaviest defeat since her formative years earlier in the tournament with a 6-0 6-2 triumph in the round-robin stage, but the world number one gained revenge in stunning fashion.
Williams' aggression was key to her play as she regularly charged into the net and attempted to intimidate Halep, who looked to engage her opponent in long rallies where possible.
After a topsy-turvy first set which featured several breaks from both players, Williams eventually took control and her dominance carried over into the second set, in which the American stormed to victory - winning all six games - to take a third consecutive Finals.
A first grand slam title at the French Open capped an impressive breakthrough 2014 for Halep, who also secured a further two titles on Tour this year, but she had no answer for a resurgent Williams.
Halep had launched out of the blocks to stun Williams in the pair's meeting earlier in the week and made a similarly swift start on Sunday, forcing the American onto the back foot.
Williams rescued two break points to hold the first game of the match, but Halep held to love before breaking to 15 - hinting that another shock was on the cards.
The aggressive approach had yet to come off for Williams, but she responded well to level the scores, climbing all over the Halep serve to break at the fourth attempt.
Five consecutive serving faults hampered Halep further and Williams emerged 5-3 from a whirlwind spell of games, before taking out the opener at the first attempt as Halep could only return a second serve into the net.
Vocal support for Halep did not appear to faze Williams, who roared her approval at taking the first set, and it may have spurred her on for what transpired in the second - a total deconstruction of her opponent's game.
The Romanian won just seven points as Williams took a mere 25 minutes to romp home.
An attempted drop shot when 5-0 and 40-0 ahead did not pay off for Williams, but a return to the power game did the trick as she forced Halep deep before lashing across court to secure the win.