Rosenthal, who returned to action after sitting out the 2018 season following Tommy John surgery in August 2017, originally signed a one-year, $7 million contract with the Nationals this off-season.
The 29-year-old reliever had a 0-1 record with a 22.74 ERA, 3.63 WHIP and five strikeouts in 6 1/3 innings over 12 appearances with the Nationals. The club released Rosenthal after he walked three batters and allowed three earned runs without recording an out against the Atlanta Braves on Saturday.
Rosenthal, who was an All-Star with the St. Louis Cardinals in 2015, rejoined the Nationals in June after beginning the season with a 36.00 ERA in seven contests. He agreed to go to extended spring training and had a 10.80 ERA across five appearances after his return.
Rosenthal spent his first six seasons with the Cardinals and served as the team's closer at times. He finished with an 11-24 record, posting a 2.99 ERA, 1.30 WHIP, 121 saves and 435 strikeouts in 325 innings with St. Louis.
Washington has an MLB-worst 6.32 ERA from its bullpen this season. The Nationals also optioned Joe Ross to Triple-A Fresno and recalled right-handers Austin Voth and Kyle McGowin in corresponding moves.