Ryan Johansen scored twice and Filip Forsberg, Matt Duchene and Nick Bonino added goals for Nashville, which finished 3-1 on a season-opening, four-game homestand.
Alex Ovechkin scored a pair of power-play goals and Lars Eller, Tom Wilson and T.J. Oshie also scored for Washington. John Carlson had three assists for the Capitals, who lost in regulation for the first time this season.
Ekholm netted the game-winner on a slap shot from the left side 28 seconds after Bonino tied the game at 5.
Ovechkin's goals were scored in nearly identical fashion coming from the left circle, a familiar spot for the Russian winger on the Washington power play.
His first came at 4:46 of the second from the top of the left circle. Ovechkin took a pass from Carlson and beat Nashville goaltender Pekka Rinne with a one-timer, his first career goal against Rinne.
Rinne finished made 21 saves.
Late in the second, Ovechkin struck again off a feed from Carlson.
Eller scored the game's first goal at 9:17 of the opening period and Forsberg tied it 1:22 later.
From the left side just inside the blue line, Duchene slipped a pass to Forsberg skating through the slot, sending him in alone against Washington goaltender Braden Holtby. Forsberg made a quick move before lifting a wrist shot over the fallen goaltender.
Holtby finished with 32 saves.
Forsberg, originally a Capitals draft pick, has a goal in each of Nashville's first four games, the first player in Predators history to do so.
Johansen tied it again at 7:37 of the second, but Washington went ahead 3-2 at 12:28 on Wilson's short-handed goal.
Notes: Rinne is 6-1-0 in his career against Washington. . Nashville RW Rocco Grimaldi missed his first game of the season due to a lower-body injury. . Capitals C Nicklas Backstrom played his 900th career NHL game. . Ovechkin has now scored against 135 different goaltenders in his NHL career.
UP NEXT:
Capitals: Visit Dallas Stars Saturday.
Predators: Visit Los Angeles Kings Saturday.
