The Royals are acquiring outfielder Blake Perkins, third baseman Kelvin Gutierrez and right-handed pitcher Yohanse Morel for Herrera, one of the most desirable bullpen arms available on the trade market.
Herrera has 14 saves and a 1.05 ERA this season for the Royals, striking out 22 in 25 2/3 innings while walking just two batters. He is coming off a 2017 season when he had a career-high 26 saves.
Sean Doolittle is having a strong season as the Nationals' closer, with 18 saves in 19 opportunities and a 1.47 ERA with 43 strikeouts in 30 2/3 innings.
Manager Davey Martinez will now have two closer-type arms to choose from at the end of games.
Gutierrez, 23, is the 10th-ranked Nationals prospect, according to MLB.com. He is hitting .274 with 6 doubles, 3 triples, 5 homers, 26 RBIs, 16 walks, 10 stolen bases and 36 runs scored in 58 games with Double-A Harrisburg.
Perkins, 21, playing in Class A Potomac, is hitting .236 with 11 doubles, one home run, 21 RBIs, 40 walks and 38 runs scored in 63 games. Morel is just 17 and was signed by the Nationals as a nondrafted free agent in 2017.