With the game tied 99-99 and time ticking down, Beal took the ball into the lane, shook off a pair of defenders and put up a soft shot from the hip that went in for the decisive score.
The Thunder's Russell Westbrook missed a shot at the buzzer, sealing a rare victory for the Wizards, who improved to 5-28 on the season and 4-13 at home.
Beal finished with 22 points, as did Martell Webster, while Kevin Seraphin added 19 as the Wizards beat the best team in the Western Conference.
Emeka Okafor provided a double-double of 12 points and 12 rebounds in the victory.
Kevin Durant netted 29 points -- including a game-tying 3-pointer 36 seconds left -- but it wasn't enough for the Thunder, who lost for the second time in four games.
Serge Ibaka added 26 points and 11 boards in Oklahoma City's loss.