Jon Rahm shot 76 on Sunday and tied for 41st, but that still was enough to clinch the season points title for the third straight year since the two-time major champion joined LIV Golf.
Niemann, who won earlier this year in South Korea, went wire-to-wire at the Trump International course in Bedminster, New Jersey.
He finished at 16-under 268 after rounds of 64, 65, 70 and his closing 69.
American Harold Varner III caught Niemann, until the Chilean pulled away late with a birdie on the 13th and an eagle on the par-5 15th.
Varner shot a final-round 70 to finish second.
The American, who catapulted into contention on Saturday with a round of 65, could not convert his chances on Sunday. He had two birdies through 11 holes, then bogeyed the 12th and made only pars the rest of the way to shoot a 70 and stay at 13 under.
Zimbabwe's Scott Vincent carded a 66 to move to nine under and tie for third with Englishman Lee Westwood (72). South Korea's Byeong Hun An (69) rounded out the top five at seven under.
"I feel like the more they push me, the better golfer I'm going to become from every experience that I experience," Niemann said.
"To be able to hit the shots that I did and make the putts that I did coming down the stretch, I put credit on myself."
Bryson DeChambeau shot 71 and tied for 38th, 23 shots behind the winner. He cannot catch Rahm with one tournament remaining.
Easing his disappointment is the fact that DeChambeau's Crushers GC won their third team title of the season.
"It's been a great year," Rahm said. "Again, a little sour taste in my mouth because of how badly I played this week, but it's been something to be proud of."
The Australian challenge fizzled in the closing rounds in New York, with Marc Leishman the best, tied for 17th at one over.
Lucas Herbert finished at two over, Cameron Smith at four over, with Travis Smyth tied for 38th place at seven over.
Sunday's final round capped a week in which LIV Golf chief executive Scott O'Neil said it has signed a term sheet with a lead investor that will allow the league to go forward without Saudi Arabia funding. O'Neil did not identify the investor or the size of the investment.
LIV Golf next plays in Indiana in a fortnight.
- with Reuters
10°C