Willett won for the seventh time in his career and moves back into the top 50 for the first time in two years.
He finished at 20-under 268. Rahm, tied with Willett going into the final round, shot 70 and finished alone in second. Rory McIlroy, who opened the European Tour flagship event with a 76, followed with rounds of 69-65-67 and tied for ninth, his 16th finish in the top 10 in 21 starts worldwide this year.
His biggest shot might have been for bogey. Willett drove wildly on the 11th, hit a tree trying to get back to the fairway, went into a bunker and finally reached the green with his fourth shot. He holed a 40-footer to escape with bogey and a one-shot lead. That was as close as Rahm got to him.
Willett still had a two-shot lead when Rahm found the water with his second shot on the par-5 18th.
Billy Horschel (65) and Patrick Reed (66) tied for fourth.
JACKSON, Miss. (AP) — Sebastian Munoz of Colombia made a 15-foot birdie putt on the 18th hole to force a playoff, and then beat Sungjae Im with a par on the first extra hole to win the Sanderson Farms Championship for his first PGA Tour victory.
Munoz, who closed with a 2-under 70, made it two straight weeks for South American winners, following Joaquin Niemann winning last week at the Greenbrier.
Im, the PGA Tour rookie of the year last season, birdied three straight holes on the back nine of the Country Club at Jackson for a 66 that looked like it might be enough for his first victory until Munoz delivered his clutch birdie.
They finished at 18-under 270. It ended a strange streak of 38 consecutive PGA Tour events without a playoff.
With the victory, Munoz earned a spot in the Masters next April. This was the first time since the tournament began in 1986 that it was not held the same week as another PGA Tour event.
Byeong Hun An birdied the last hole for a 69 to finish alone in third. An was tied for the lead until making bogeys on the 13th and 14th holes.
PGA TOUR CHAMPIONS
SIOUX FALLS, S.D. (AP) — Rocco Mediate birdied his last two holes for a 6-under 64 and won the Sanford International on Sunday when Ken Duke took double bogey on the final hole.
Mediate won by two shots for his first PGA Tour Champions title in three years.
Duke was tied for the lead until taking four shots to reach the green on the closing hole at Minnehaha Country Club. He closed with a 69 and tied for second with Colin Montgomerie and Bob Estes, who each shot 67.
Mediate finished at 9-under 201. It was his fourth PGA Tour Champions victory, and first since the Senior PGA Championship in 2016.
Duke started the final round tied for the lead with Kirk Triplett, who faded to a 71.
One week after going unbeaten in the Solheim Cup, Nelly Korda closed with a 4-under 67 for an eight-shot victory in the Lacoste Ladies Open de France for her first title on the Ladies European Tour. ... Women's British Open champion Hinako Shibuno closed with an 8-under 64 for a two-shot victory in the Descente Ladies Tokai Classic on the Japan LPGA Tour. ... Laura Restrepo of Panama closed with an 8-under 64 to win the Guardian Championship on the Symetra Tour by one shot over Jenny Coleman and Gigi Stoll. It was Restrepo's first top 10 of the season on the Symetra Tour, which has two tournaments remaining. ... Jbe Kruger of South Africa closed with a 6-under 65 and won the Shinhan Donghae Open on the Asian Tour by two shots over American Chan Kim, who also had a 65. ... Chandler Blanchet closed with a 2-under 69 for a one-shot victory over Augusto Nunez in the Sao Paulo Golf Club Championship on the PGA Tour Latinoamerica. ... Motin Yeung of Hong Kong shot a 5-under 67 for a one-shot victory over Peter Campbell of Canada in the Zhuzhou Classic on the PGA Tour China Series. ... David Shacklady closed with a 4-under 68 and won with a par on the first playoff hole over Jean-Francois Remesy (67) to win the Paris Legends Championship. It was Shacklady's second straight victory on the Staysure Tour. ... Hee Jeong Lim closed with a 1-under 71 and won the All For You Championship in a playoff over Ji Hyun Kim on the Korean LPGA Tour.