The American closed the gap on series leader and countrywoman Gwen Jorgensen ahead of the Grand Final in Edmonton, Canada, with an impressive performance.
Groff accelerated away from the lead group on the first lap of the run and had opened up a four-second advantage by the time she crossed the line in a time of one hour and three minutes.
Victory moves Groff above Great Britain's Jodie Stimpson and 848 points behind Jorgensen overall, with 1000 points up for grabs in the final event of the season.
New Zealand pair Andrea Hewitt and Nicky Samuels shared a sprint for the line, with Hewitt just edging her compatriot as the duo completed the podium places.
They had surged away from Canada's Kirsten Sweetland in the final straight, with Stimpson having to settle for 16th having been set back by a poor swim.
Groff was only bettered by Spaniard Carolina Routier out on the swim but was part of a large lead group that began to pull away on the bike phase around the damp course.
Having finished the ride in the pack, the American then seized the initiative on the run to assert her authority.
With Jorgensen not competing in Stockholm, her lead was always going to be cut but with Stimpson failing to claim big points she remains favourite to claim the title heading into the Grand Final.