McCoy played the entire game, completing 22-of-29 of his passes for 321 yards with two interceptions for the Redskins. Ryan Grant and Lache Seastrunk each had a receiving touchdown for Washington in the preseason finale for both teams.
Silas Redd rushed for a game-high 79 yards and a touchdown, while Seastrunk caught two passes for 88 yards and rushed for 23 more yards for the Redskins, who finished 3-1 in the preseason.
Washington will open the regular season in Houston on Sept. 7.
Quarterback Mike Kafka went 7-of-14 for 86 yards and a score and rushed for 22 yards and Mike Glennon completed two of his three passes for just 10 yards,
Jeff Demps led Tampa Bay rushers with 59 yards on 20 carries and rookie receiver Solomon Patton caught two catches for 34 yards and a score.
The Buccaneers, who went 1-3 in preseason play, will host Carolina on Sept. 7 to start the season.
Kai Forbath opened the scoring with a 45-yard field goal with 40 seconds remaining in the first quarter. After Kafka fumbled the ball, McCoy found Grant from two yards out for a 10-0 edge early in the second.
Patrick Murray made a 35-yard field goal to get Tampa Bay on the board early in the third, but McCoy responded with an 80-yard touchdown pass to Seastrunk.
Patton caught a 25-yard pass to make it a one-score contest with under four minutes to play in the third, but Redd plunged into the end zone from a yard out for a 24-10 advantage with four minutes to play, sealing the win.