Ribeiro put in the puck after Troy Brouwer deflected a shot from the blue line.
The Capitals lead the series 3-2. Game 6 is Sunday in New York.
The home team has won every game in the series, with Washington taking the first two and New York pulling even in Games 3 and 4. The Capitals have been fighting history along the way: The franchise has lost eight playoff series in which it has held a two-game lead.
I think the thing about him, No. 1, he's a competitive guy. We have no issues as far as him fitting in here with this group. No question he's going to be hungry. I think whenever you look at your back end, it's a mix-type thing, and I think [there are] things he can bring to us. He's very mobile, and even though he's not big (6 feet, 201 pounds), he certainly competes. He's a smart player.
— Kings general manager Dean Lombardi on Andrej Sekera, after making a trade with the Hurricanes for the defenseman on Wednesday