Antonio Daniels added 18 points off the bench for Washington.
Arenas, who came into the game 8-for-43 on three-point tries in his previous five games, made all five attempts from beyond the arc, on the way to a 9-for-13 shooting night.
Chris Bosh scored 25 points, but had only one rebound for the Raptors, who lost their third straight. T.J. Ford and Juan Dixon scored 14 points each for Toronto, and Jose Calderon added 13 points off the bench.
Arenas scored 11 straight Wizards points to extend Washington's lead to 86-64 with 8:25 left in the third quarter.