The Nationals were unavailable for comment.
Washington is in need of bullpen help. This past season, Nationals relievers posted a 5.09 ERA, which ranked 30th in the Majors.
Capps has spent his entire five-year career with the Pirates, collecting 67 saves while posting a 3.61 ERA. He was non-tendered by Pittsburgh on Dec. 12. Capps will battle Brian Bruney for the closer role in D.C.
Capps, 26, picked the Nationals over the Cubs because general manager Mike Rizzo showed serious interest the moment Capps became a free agent. Washington also has told him he would get a chance to close games.
"[I liked the Nationals because of] the way Mr. Rizzo treated me throughout this whole process," said Capps. "The Nationals organization has been first class.
"They expressed their interest from Day One -- the moment I was non-tendered. They wanted me to become a Washington National. That means a lot. The feeling of being wanted is very important. I felt that from them from Day 1.
"They are giving me an opportunity to close out the ballgame, which was important to me," Capps added. "I feel that is an opportunity I'm going to have. It's going to be a lot of fun in Washington. I'm looking forward to 2010 and beyond. ... If I pitch the way I know I can pitch and the team has the success they should have, it's going to be a lot of fun."
Capps becomes the third ex-Pirate acquired by the Nationals this year. On June 30, Rizzo acquired center fielder Nyjer Morgan and reliever Sean Burnett from Pittsburgh in exchange for outfielder Lastings Milledge and right-hander Joel Hanrahan.
That deal is considered by many to be a steal in favor of the Nationals. Morgan is a solid defensive player and leadoff man, while Burnett gave Washington quality innings in a setup role.
"I already texted [Morgan and Burnett] I would be there," Capps said. "I talked to both of them a lot during the process. At the same hand, I talk to John Grabow and Tom Gorzelanny with the Cubs. They were another team that was interested in me. It was a win-win situation for me. I felt either way I went, it would be in a good situation.
"But I really like the decisions the Nationals have made this offseason and their commitment to winning beyond 2010. I'm excited about what can be."
Capps said he is dedicating the 2010 season to his father, Mike, who passed away unexpectedly in October. Matt acknowledged he thought about his father during the free-agency process.
"My dad gave me my work ethic and he gave me a lot of opportunities in life. He gave me the drive," Capps said. "For the last eight years since I've been in pro baseball, I talked to him every single day -- sometimes 10 times a day. I'm going to miss him. This whole [free-agency] process was very difficult for me because I can't tell you how many times I picked up the phone and got ready to dial his number. It was tough for me."