The deal, which is pending a physical exam, is worth about $2 million, a baseball source said.
Beimel, 31, has a 21-23 record in eight seasons with the Pittsburgh Pirates, Minnesota Twins, Tampa Bay Rays and Los Angeles Dodgers. He's allowed only one home run in 154 appearances over the last two seasons.
Beimel is also known for holding Barry Bonds to an .063 career average, with one single and three walks in 19 plate appearances.
The Nationals had been looking for a veteran arm to add depth to their bullpen and take the pressure off a young starting rotation, which is led by John Lannan, Scott Olsen, Daniel Cabrera, Shairon Martis, Collin Balester and top prospect Jordan Zimmermann.
Jerry Crasnick covers baseball for ESPN.com.