Arroyo has agreed to a minor league contract with an invite to Major League Spring Training. He will be reunited with Dusty Baker, who managed Arroyo for most of his career while he pitched for the Reds.
If he makes the Nationals' 25-man roster, Arroyo will earn a $2 million base, according to Mark Sheldon of MLB.com. He can make up to $8 million if he makes 32 starts.
Washington had been mentioned as a suitor for Arroyo just this week, with the Reds, Marlins, and Padres also showing varying amounts of interest.
With the deal, Arroyo adds depth to the Nationals' rotation that includes Stephen Strasburg, Max Scherzer, Gio Gonzalez, Tanner Roark, and Joe Ross.
Turning 39 this February, Arroyo has not pitched since 2014, having had Tommy John surgery. This past season, Arroyo was traded from the Diamondbacks to the Braves to the Dodgers, but did not throw a single pitch for any of them.
Arroyo's last full season came in 2013, while with the Reds. He went 14-12 with a 3.79 ERA and a 124 to 34 strikeout-to-walk ratio in 202 innings pitched.
Over his career, Arroyo was known as one of the most durable pitchers in the Majors. He threw 199 or more innings every season from 2005 to 2013, with solid results during that time. Overall, with the Reds, Pirates, Red Sox, and Diamondbacks, he has a 4.19 ERA and a 2.38 strikeout-to-walk ratio in 2,364 2/3 innings.