The Wizards won for just the eighth time all season and picked up only their second road victory in 18 tries. Denver has lost two in a row after winning six straight.
Washington trailed by seven heading into the fourth quarter, but Ariza hit a 3-pointer in sparking the Wizards to the first 12 points of the period.
Jordan Crawford and Bradley Beal both could make just 1-of-2 foul shots in the final 21 seconds, but Denver missed its last two shots from the field and John Wall's two free throws with 2 seconds remaining clinched the victory.
Beal scored 23 points to pace the Wizards and Kevin Seraphin added 18. Washington has won four out of five after capturing only four of their first 32.