Patrick Cantlay birdied the 18th hole at East Lake to win the Tour Championship, FedEx Cup title and $15m (£10.9m) first prize in Atlanta.
The American hit a closing one-under-par 69 to finish on 21 under, one shot ahead of world number one and US Open champion Jon Rahm, who shot a 68.
"It felt huge and it was," he said.
"We distanced ourselves from the field and it was like a one-on-one matchplay feel, and I think it will be great practice for the Ryder Cup."
Cantlay's win at last week's BMW Championship earned him one of six qualification spots in the US team at Whistling Straits for the 43rd Ryder Cup from 24-26 September. Captain Steve Stricker will name his six wildcard picks on Wednesday, 8 September.
Rahm is set to be in the European team, with captain Padraig Harrington confirming the nine qualifiers and his three wildcard picks after the conclusion of the European Tour's BMW Championship at Wentworth on Sunday, 12 September.
Victory at last week's BMW Championship in the PGA Tour's end-of-season play-offs, also saw Cantlay move top of the Fedex Cup standings and meant he started this season-ending tournament on 10 under par, two shots clear of his nearest rival under a controversial system adopted in 2019.
Cantlay kept that two-shot advantage after round one but saw it cut to one by Rahm after round two.
He restored his two-shot lead over Rahm heading into the final round, with Justin Thomas a further three shots back but the race for the title was always going to be between the top two.
Cantlay moved three ahead with a birdie on the second but Rahm closed to within one with a birdie on the fifth as the American had a three-putt bogey.
That was as close as the Spaniard got though. He was again within one when Cantlay bogeyed the 17th but his birdie on the par-five closing hole was matched by the American who held on for the biggest win of his career.
"I just kept telling myself to focus and lock in and I did a great job of that," said the world number four. "It was definitely different than any other week. It was the longest lead I've ever held.
"But I just tried to stay, day after day in the present, and I did an amazing job of that this week because the last couple days I made some mistakes I don't usually make and I was able to really centre myself and hit a lot of good shots when I needed to."