Jade Zierden got the scoring started at 12:16 of the opening period, assisted by Ellie Andersen and Gabby Brimhall.
Khloe Lund tied the game at 2:30 of the second period with an unassisted goal, however, Fergus Falls regained the lead when Ellie Andersen converted a power play goal from Zierden.
A Jadyn Bye goal, assisted by Brimhall and Hope Goepferd at 4:30 of the third gave the Otters a 3-1 lead. Thief River fought back to within a goal when Shelby Breiland scored from Lexi Huot and Jenae Johnson at 5:40 of the period and then tied the game at 12:10 when Bella Amiot scored from Huot.
The winning goal came at 5:26 of overtime, a shorthanded tally by Breiland.
Thief River Falls goalie Bell Hamre stopped 24 shots and Fergus Falls goalie Lexi Metcalf made 20 saves on the night.
Fergus Falls (9-11) will play at Alexandria on Tuesday night.