The USA looked headed for more disappointment against their arch rivals when trailing late in the third period, but captain Hilary Knight tied it to take the Games decider to three-on-three sudden death hockey, where Keller went around defender Claire Thompson and slipped the puck past the Canadian goalie.
The game at Santa Giulia Arena gave fans yet another classic chapter in the sport's greatest rivalry, as fears of a one-sided final were soon dispelled when Canada took the lead, having lost 5-0 to the USA in the group stage.
This marked their seventh successive loss to the Americans, and the third Olympic gold for the US women.
The USA began the second period on a power play carried over from the first, only to see the Canadians strike shorthanded. Laura Stacey gathered the puck in the neutral zone, charged forward and sent it across for Kristin O'Neill, who made a nifty backhand move to score.
Finding themselves behind for the first time in the tournament, the US could not find a way past Canada's goalie Ann-Renee Desbiens until with just over two minutes left and the US net empty for an extra skater.
Knight redirected a Laila Edwards shot for their desperation first goal, and Keller finished off the Canadians with just over four minutes gone in overtime.
The USA were playing in their seventh Olympic championship game, having missed just one. Switzerland took the bronze medal earlier on Thursday with a 2-1 overtime win over Sweden.