After finishing second behind Jason Day at the 2015 PGA Championship, he became the world number one in the Official World Golf Ranking. He was the 18th different golfer to earn the honor. He was number one for two weeks in August 2015 and one week in September.
Spieth missed the cut in The Barclays and the Deutsche Bank Championship, the first two events of the FedEx Cup playoffs. However, his tied for 13th finish at the BMW Championship kept him second overall in the standings. Only needing a victory to clinch the championship, Spieth won the 2015 Tour Championship at East Lake Golf Club by four strokes. With the win, his fifth of the year, Spieth became the ninth FedEx Cup champion and earned a $10 million bonus for winning the Cup. Spieth won $12,030,485 (not including the $10 million bonus) in 2015, a PGA Tour record for a single year. He regained the world number one ranking.
A month away from the Masters Tournament, Spieth was hoping to kick-start his season at the Valspar Championship, where he was victorious in 2015, which led on to his magnificent year with his win at Augusta, as well as the U.S. Open. However, Spieth's difficult campaign continued, missing the cut at +5. His next start at the WGC Match Play saw him seeded 4th. He was victorious over Charl Schwartzel and Li Haotong, but was defeated by Ryder Cup playing partner Patrick Reed in their winner-takes-all final match, which meant Spieth failed to progress out of the group stage. In his final event before Augusta, Spieth finished in a tie for 3rd at the Houston Open.