Soriano, 33, had one of his best seasons in 2012, posting a 2.26 ERA with 42 saves for the Yankees, taking over the closer duties after Mariano Rivera's injury. He joins a Nats bullpen that already includes Drew Storen and Tyler Clippard, who closed games in 2012.
Soriano has a 2.78 career ERA in 11 seasons with the Mariners, Braves, Rays and Yanks.
The signing of Soriano comes after Washington's bullpen imploded during the National League Division Series, allowing 16 earned runs against the Cardinals. The worst performance came in the ninth inning of Game 5, when Washington held a 7-5 lead going into the top of the ninth inning before Storen allowed four runs for a 9-7 loss.
Storen was one strike away from getting David Freese for the final out, but Freese walked. Daniel Descalso came to the plate, and on the first pitch, he singled off the glove of shortstop Ian Desmond, scoring Carlos Beltran and Adron Chambers to make it a 7-7 game.
Manager Davey Johnson then elected to pitch to Pete Kozma, rather than walk him to get to pitcher Jason Motte. Kozma singled to right field to send home Freese and Descalso to put the Cards on top.
With Soriano on board, the Nationals most likely will trade Clippard or Storen. Before the non-waiver Trade Deadline in 2011, the Nats tried to trade Storen to the Twins for Denard Span, but that deal fell through because Span suffered a concussion that forced him to miss a lot of action. Minnesota later traded Span to Washington for pitching prospect Alex Meyer last December.
Storen missed half of the 2012 season because of bone chips in his right elbow. He returned to action after the All-Star break but didn't reclaim his role as the closer until September, when Clippard slumped.
Nationals general manager Mike Rizzo could try to package Storen and Michael Morse in a deal for prospects.
The Nats' Major League roster is nearly complete. They are looking to sign a starting pitcher, even though their rotation is set, and they continue to have interest in right-hander Javier Vazquez, who has not pitched in the Majors since 2011 and has not yet decided if he will play in 2013.