Ethier hit a 1-2 pitch out to right off Rafael Soriano (1-2), bringing the Dodgers their seventh victory in nine games. The other Los Angeles runs came in the third on a homer by Hanley Ramirez against Stephen Strasburg.
Ethier's blast made a winner out of Ronald Belisario (4-5), who faced only one batter in the eighth inning and retired him on two pitches.
Kenley Jansen struck out two in a perfect ninth for his 10th save.
The Washington runs scored on a wild pitch in the second inning and an RBI single by Ian Desmond in the sixth.