The 33-year-old DeJesus is a career .279 hitter with 86 homers and 513 RBIs in 11 seasons with Kansas City, Oakland and Chicago. He can play all three outfield spots.
"He's a great addition," Nationals manager Davey Johnson said. "He's a quality player. Anybody gets hurt, he can step right in and play at a high level."
DeJesus was one of the first acquisitions for the Cubs after Epstein became the president of baseball operations. He agreed to a $10 million, two-year contract in November 2011.
The Cubs get a player to be named in the trade with the Nationals, but they also save about $2.5 million, including a $1.5 million buyout for a 2014 team option worth $6.5 million. General manager Jed Hoyer said the money was a "strong consideration."
"We really like David a lot," Hoyer said. "I've told his agent and Theo told David directly, we'd be willing to talk to him about bringing him back at some point. He's a good mentor for our younger guys."
The Cubs activated outfielder Brian Bogusevic from the 15-day disabled list to take DeJesus' roster spot, and Hoyer said outfielder Ryan Sweeney likely will begin a rehab stint next week. Bogusevic had been out with a strained left hamstring, and Sweeney is recovering from a broken left rib.
The Nationals released outfielder Roger Bernardina to make room on their roster. Bernardina hit .178 in 85 games with Washington.
Jay Cohen can be reached at http://www.twitter.com/jcohenap