Miami, which rested both reigning NBA MVP LeBron James as well as All-Star center Chris Bosh in its second-to-last game prior to the playoffs, surrendered the Eastern Conference's top seed to the Indiana Pacers with its fourth loss in five outings.
The Heat allowed the Wizards to shoot a robust 59 percent from the field and go 14-of-29 from 3-point range, with Ariza knocking down 5-of-8 trey attempts and Al Harrington hitting 4-of-6 from beyond the arc in a 16-point effort off the bench.
Bradley Beal contributed 15 points to Washington's third consecutive win, with Marcin Gortat posting a double-double of 10 points and 13 rebounds.
Michael Beasley paced the shorthanded Heat with 18 points and Toney Douglas scored 14 in defeat. Dwyane Wade, playing for the second time since returning from a nine-game absence caused by an injured hamstring, had just nine points in 18 minutes.