The win put the defending Stanley Cup champions on the verge of surviving the first round, leading 3-2 heading to Game 6 in Los Angeles on Saturday.
Alex Pietrangelo scored on a wrist shot from the point with 44.1 seconds remaining in regulation and goalie Brian Elliott off for an extra attacker. That forced overtime for the second time in the series.
The Kings' Justin Williams scored in the final minute of Game 1, also in St. Louis.
Jeff Carter scored in the opening minute of the second and third periods for the Kings, who became the first road team to win in the series.
I don't know how he does it. I don't know how he gets his body parallel with the player and pulls it through his legs like that. I know he's tried it a couple times in practice and it's never worked, so how he does it in a game, it's incredible.
— Capitals defenseman Mike Green on teammate Alex Ovechkin's highlight-reel goal against the Devils on Saturday