The 29-year-old right-hander was released on Sunday, according to a team announcement.
Rosenthal, who was once an effective closer with the St. Louis Cardinals, struggled during his first season with the Nationals, posting a 22.74 ERA with 15 walks in 6 1/3 innings.
"We put the work in, he put the work in. It just didn't work out," Nationals manager Dave Martinez said following the release, according to MASN's Mark Zuckerman. "Time for us to move on."
The Nationals signed the former All-Star to a one-year, $7-million deal in November after he missed all of 2018 due to Tommy John surgery.
But while Rosenthal did not pitch well for Washington, his troubles weren't due to velocity, as he averaged 98 mph on his fastball.
Earlier this season, Rosenthal tied an MLB record by making five consecutive appearances without recording an out.
The veteran hurler has been one of the many disappointments for a Washington bullpen that ranks last in ERA (6.32) and opponents' batting average (.272) this season.
In a corresponding move, the team recalled right-hander Austin Voth.