Spieth, winner of the opening two majors of the season, carded a final round one-under 69 at East Lake for nine-under 271 to finish four shots shots clear of Sweden's Henrik Stenson, Danny Lee of New Zealand and England's Justin Rose.
Victory was worth a cool $11.485 million, with the $10 million bonus for the FedEx Cup victory.
Australia's Day needed to finish in the top five to retain his status, but a two-under 69 for two-under overall was only good enough for a tie for 10th.
The Texan had battled all round with Stenson, the FedEx Cup winner from 2013, but strong play around the turn -- where he carded three birdies in four holes -- took him clear.
Stenson's challenge was finally over when he double bogeyed the 17th and it needed a brave birdie on the closing hole to see him join Lee and Rose at five-under 275
Lee and Rose of England had come out of the pack with fine closing rounds of 65 and 66 respectively, but neither was able to seriously challenge for victory.
Best round of the day was Dustin Johnson's 64 to take him into a share of fifth with Bubba Watson and England's Paul Casey, a further shot back.
Northern Ireland's Rory McIlroy, who had been top of the rankings for most of 2015 before an ankle injury disrupted his season, slid to a disappointing four-over 74 to finish on one-over par 281 and tied 16th.
But this was all about the 22-year-old Spieth, who was winning for the fifth time in a remarkable season, which has catapulted him to international prominence.
He missed the cut at the first two FedEx Cup events in a rare lapse, but has battled back in typical style to claim the prestigious end of season honor.
"This is unreal. It really is. This year is unreal," Spieth said at the victory presentation.