Washington also claimed an Eastern Conference playoff berth after
Florida could have wrapped up their first division title with a victory over Washington and will need at least a point in their regular season finale against Carolina to make sure of the Southeast crown.
"It doesn't matter how you get in as long as you get in," Florida defenseman Brian Campbell said about qualifying for the postseason.
Washington, who made the playoffs for a fifth consecutive season, came into Thursday's game deadlocked with Buffalo on 88 points for the East's final playoff spot, but can still win the Southeast if Florida lose in regulation and the Caps beat the New York Rangers on Saturday.
The Capitals raced to a 3-0 lead on goals from Jay Beagle, Alex Ovechkin and Brooks Laich before Florida fought their way back with strikes from Mikael Samuelsson and Ed Jovanovski.
Alexander Semin iced the win for Washington late in the third period.
"We're watching the scoreboard when we're up 3-0 and (Buffalo) is up 1-0 and you're like, 'C'mon,'" Capitals forward Jason Chimera said.
"Then all of a sudden, (Philadelphia) was up 2-1 and it was like, 'Geez, we've got to get this done.' The intensity picked up a little from there."
It was not all good news for the Capitals, however, after netminder Michal Neuvirth left the game in the second period with a left leg injury.
In Philadelphia, Matt Read ended Buffalo's playoff hopes on a breakaway goal with 4:21 left in the third to break a 1-1 tie.
Ville Leino scored the Sabres' goal in the second period before Marc-Andre Bourdon tied the score at 1-1.
Ryan Miller had 33 saves for Buffalo.
Also in the East, the
In the Western Conference, the
The Sharks gained the West's seventh playoff spot with the win, and the Phoenix Coyotes took the eighth and final berth.