The American has won two majors this year.
Jordan Spieth is the new world no. 1 after his second placed finish at the USPGA Championship.
The American replaces Rory McIlroy at the summit of the Official World Golf Rankings, with McIlroy’s 17th placed finish not enough to keep him on top.
Spieth finished four shots behind tournament winner Jason Day after a final round 68, making six birdies and two bogeys.
WATCH: Jason Day’s swing sequence
Speaking after Sunday’s round at Whistling Straits, Spieth said: “That will never be taken away from me now. I’ll always be a No. 1 player in the world.
“When I look back on this year, the consistency that we have had this year and especially being able to step it up in the biggest stages, that’s a huge confidence builder and that’s what’s allowed us as a team to become the best, the No. 1 ranked, I should say, and I believe right now the best in the world.
“Second best behind Jason Day, of course, given this week,” he added.
Spieth won the Masters and the US Open earlier this year, winning by four shots from Phil Mickelson and Justin Rose at Augusta National and one from Louis Oosthuizen and Dustin Johnson at Chambers Bay.
Wins at the John Deere Classic and Valspar Championship have helped Spieth climb the leaderboard to now overtake McIlroy, who missed the Open and WGC Bridgestone Invitational through injury.
Jason Day is now third in the rankings after his win in Wisconsin, with Justin Rose and Dustin Johnson also climbing one place respectively.