The Heat rested all its key players for the upcoming post-season grind and found themselves 25 points behind at halftime.
Washington had just 20 victories this season, but finished the campaign on a six-game winning streak. Kevin Seraphin and Nene both had 15 points for Washington.
Miami shot 35 percent for the game and was led by Norris Cole with 14. LeBron James, who sat out the contest along with Chris Bosh and Dwyane Wade, concluded the regular season averaging 27.3 points per game.
As the No. 2 seed in the Eastern Conference, Miami will open the playoffs Saturday against New York.