Spieth, who had a remarkable run in the majors that saw him win the Masters and U.S. Open and miss a playoff by one stroke at The Open, finished three strokes back of PGA Championship winner Jason Day.
The solo second-place finish coupled with Rory McIlroy's 17th-place finish meant a new No. 1 for the first time in more than a year.
At 22, Spieth becomes the second-youngest No. 1. Tiger Woods was 21 when he went to No. 1 in 1997.
Spieth has earned over $10 million this year and collected a cool $1.1 million with Sunday's second-place finish.
Spieth needed at worst a two-way tie for second if McIlroy was worse than a three-way tie for 13th.
"The way Jordan has been playing and the way I haven't played much this year ... if he goes to No. 1 today, it's very deservedly so," said McIlroy, who shot a final-round 69 and finished some 90 minutes before the final group. "Winning two majors, winning a couple other times this year, had a chance at The Open, has a chance obviously today...
"And if he was to get to No. 1 today, I'd be the first one to congratulate him because I know the golf you have to play to get to that spot, and it has been impressive this year."
McIlroy, 26, went to No. 1 in the world for the second time in his career after capturing the 2014 WGC-Bridgestone Invitational. That was the second of three straight victories that included The Open and the PGA Championship.
This year he won the European Tour's Dubai Desert Classic as well as the WGC-Match Play and the Wells Fargo Championship. He was a solid No. 1 at the time, but Spieth not only won the Masters and U.S. Open, but his victories at the Valspar Championship and John Deere Classic closed the gap.
Starting with the Memorial Tournament in June, Spieth has not been out of the top four. He ended 2014 ranked No. 9 in the world.
"I accomplished one of my life-long goals and in the sport of golf,'' Spieth said. "That will never be taken away from me now. I'll always be a No. 1 player in the world. That's what, 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, and I believe right now, the best in the world.''
McIlroy, who played his first tournament at the PGA since suffering an ankle injury on July 4 that kept him out of three events, will have plenty of opportunities the rest of the year to overtake Spieth.
He is scheduled to play three of the four FedEx Cup playoff events, the Frys.com Open on the PGA Tour in October, then likely three of the four Final Series events on the European Tour, including the season-ending event in Dubai. Spieth is also scheduled to defend his Australian Open title in November and is expected to defend his Hero World Challenge title a week later.
"I've always said that winning golf tournaments takes care of all of that stuff," McIlroy said. "Right now I'm focused on just getting my game the way I think it has to be to win tournaments like this. I'm happy with where my game is considering all I've had to go through to get back here. And just looking forward to the rest of the season."
Information from ESPN's Darren Rovell contributed to this report.
Spieth: 'Best loss I've ever had'
Jordan Spieth addresses the media after taking over the No. 1 world golf ranking with a second-place finish at the PGA Championship.