Niemann carded a three-under 68 on Sunday to finish with a 17-under 196 for the tournament at the JCB Golf and Country Club in Rocester, England.
The Torque GC captain collected five birdies against two bogeys to add his to his titles at Adelaide, Singapore, Mexico City and Virginia.
Sunday's victory served as a $4 million payday for Niemann, who began the week by firing his caddie and coach after missing the cut at the 2025 Open Championship.
Niemann's Torque GC, however, were unable to hold their three-stroke lead over Legion XIII in their bid for a first team title since 2023.
Legion XIII shot a staggering 14-under on Sunday to finish at 35-under for the tournament, eight strokes better than Torque GC.
Niemann wasn't too interested in talking about his individual play on Sunday.
He said: "Well, I'm going to talk for the first two days because today I didn't feel like I had my best. Yeah, it was great. I was able to flow on tough situations.
"I felt like today I was pretty calm, pretty chill, until Bubba (Watson) started playing golf. He made it tough for me. I was actually feeling the pressure on 13, 14, but, yeah, just stay in the present. Yeah, was able to hit a great shot on 15. After that put me back into my place."
Watson had a stretch of four birdies and two eagles from No.9 to No.14. He rolled in six birdies overall to finish in second place at 14-under after a third-round 65.
Niemann put a halt to his charge after sending a shot within five feet of the cup on the 15th hole.
"I was pretty happy to see that ball go in that close, and making that birdie I think was huge for the next three holes," he said.
Caleb Surratt (65 on Sunday) sank six birdies during his bogey-free round to finish at 13-under for the tournament, followed by Talor Gooch (66) at 11-under.
As for team play, Legion XIII captain Jon Rahm was thrilled after his team captured their fourth title of the season.
"With all four scores counting this year I thought we played to our advantage. It showed early on and it's showing now," Rahm said.
Cameron Smith was the highest placed Australian, finishing 10th, nine shots off the lead. Marc Leishman was a shot behind with Lucas Herbert 34th and Matt Jones 40th.
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