Haren, 32, broke into the majors in 2003 with the St. Louis Cardinals and played the last three seasons with the Los Angeles Angels. He has a 119-97 record and 3.66 ERA over 10 seasons.
Haren was 12-13 with a 4.33 ERA last year as he worked around back injuries. He worked at least 216 innings each year from 2005-11 but was limited to 176.2 innings last season.
He was named to the All-Star Game three times, in 2007 and '08 while with Oakland and 2009 while with Arizona.
The signing of Haren -- to a one-year deal reported to be worth $13.5 million -- enhances an already strong Washington starting rotation. The Nationals' Stephan Strasburg, Gio Gonzalez and Jordan Zimmerman combined for 47 wins last season.
Gonzalez and Zimmerman are both 26 while Strasburg is 23.