Finau, 33, has now won four PGA Tour events in his past 18 starts, and all in under a year's time.
The American won the 3M Open back in late July 2022 for his third career victory and followed it up a week later with another W at the Rocket Mortgage Classic in Detroit.
He continued his impressive end to last season with a quick start to the 2022-23 campaign, winning the Houston Open in November.
And on Sunday in Mexico, he shot a final-round 5-under par 66 to defeat Rahm by three strokes at 24-under par to earn his sixth career victory on the PGA Tour.
Finau entered the round with a two-stroke lead over the Spaniard, who already has four wins this PGA Tour season, including The Masters earlier in April.
And despite closing with a 4-under par 67, it wasn't enough to defend his lone PGA Tour win from the 2021-22 season. He was even seen throwing shade towards his caddie as he struggled to cut into Finau's lead.
Following his final round Sunday, Rahm admitted he needs a bit of time to rest before the PGA Tour heads to the second major championship of the 2023 season in a few weeks with the PGA Championship beginning 18 May in Rochester, New York.
Rahm has decided to skip his first designated event of the PGA Tour season at next week's Wells Fargo Championship.
He told reporters:
Brandon Wu finished third at 19-under par while special temporary PGA Tour member Akshay Bhatia finished six strokes back at 18-under par in fourth.
With the win Sunday at the Greg Norman-designed Vidanta Vallarta course, Finau jumps up seven spots in the FedEx Cup standings from No. 11 to No. 4.
Next week, the PGA Tour will head to Quail Hollow Club for the Wells Fargo Championship where Max Homa will defend his title at the latest designated event with $20m on the line.
Mexico Open Final Leaderboard
-24 T Finau
-21 J Rahm
-19 B Wu
-18 A Bhatia
-16 E Grillo, E Cole, A Smotherman