The six-game skid matches the Flyers' franchise record. It's the second time this season they've dropped six in a row; each streak includes an overtime defeat.
Forsberg's return from a four-game absence provided the Flyers with a momentary lift. After finally finding a skate to accommodate his surgically repaired right ankle, the veteran centre received an assist on a goal by Mike Knuble that put Philadelphia up 1-0 at 3:31 of the first period.
But the lead vanished by the end of the period, and so did Forsberg. He missed the final 40 minutes with a concussion.
Semin assisted on Dainius Zubrus' first-period goal, then scored twice in the second period to put Washington ahead 3-1.
Ovechkin, who had a hat trick one night earlier in Atlanta, scored with 16:44 left to make it 4-1. He has 22 goals and 21 assists in 32 games.
Washington has won seven of nine, including two victories over Philadelphia. Olaf Kolzig had 21 saves for the Capitals, who are 3-0 against the Flyers this season.