Spieth, the Masters and US Open winner, was touted as the player headed for history at the 97th PGA Championship, where he had a chance to join legendary Ben Hogan and superstar Tiger Woods as the only men to win three majors in a year.
But Day thwarted that dream with a record-setting 20-under par winning total for a three-stroke win over Spieth, who nevertheless dislodged Northern Ireland's Rory McIlroy from the number one spot he had held for 55 consecutive weeks.
"That will never be taken away from me now. I'll always be a number one player in the world," added the American, the second-youngest to ascend the summit behind Woods, who was 21 when he first reached number one in 1997.
Despite the outcome on Sunday, Spieth has produced a remarkable 2015 campaign, capturing the Masters and US Open and missing a playoff by one stroke at the British Open to finish equal fourth before his runner-up effort at Whistling Straits.
"The way Jordan has been playing and the way I haven't played much this year ... If he goes to number one today, it's very deservedly so," said McIlroy, who walked off the course 90 minutes before the last putts fell.