Washington general manager Mike Rizzo announced the trade Tuesday, a week before pitchers and catchers report to the team’s first spring training in West Palm Beach, Florida.
Romero, 26, had a 5.91 ERA with 50 strikeouts and 28 walks in 45 2/3 innings over 52 appearances last season, his first in the majors. His fastball averaged 96.1 mph, which ranked ninth among American League relievers.
In 76 major league appearances, Romero is 2-2 with a 5.27 ERA.
Rosa, 21, went 1-8 with a 3.33 ERA in 24 starts for the Dominican Summer League Nationals and Gulf Coast Nationals over the past two seasons.