Soriano, who saved 42 games for the Yankees last season and 45 for the Rays in 2010, joins fellow right-handed relievers Drew Storen and Tyler Clippard, who both have 30-plus-save seasons in the majors.
"You strengthen a strength," Nationals general manager Mike Rizzo said at a news conference Thursday, describing the addition of the 33-year-old Soriano to the reigning NL East champion's bullpen.
"We feel that we have multiple closers on this club that have the ability to close out games, and one of them is going to close out the seventh, one will close out the eight and one will finish the game in the ninth," Rizzo said. "We feel pretty good about that."
And Rizzo left no doubt that Soriano will be the top choice.
"Suffice it to say," the GM said, "Raffy is here to pitch the ninth inning and he has done it successfully everywhere he has been and we expect him to continue that."
Storen had 43 saves in 2011, but missed the first half of last season after an elbow operation in April. He failed to hold a ninth-inning lead against St. Louis in Game 5 of the NL division series, giving up four runs in a 9-7 loss that ended Washington's season.
"Drew Storen is a closer. He's going to be a closer. He's got closer stuff. He's got a closer mentality," Rizzo said. "And by no means (was) the signing of Rafael Soriano ... based on one inning and one game at the end of the season."
Rizzo said he has yet to speak to Storen or Clippard, who saved 32 games in 2012, but pitching coach Steve McCatty did.
"I'm certainly not worried about Clip or Drew," Rizzo said.
A one-time All-Star, Soriano had 42 saves in 46 chances and a 2.26 ERA for the AL East champion Yankees last year while filling in for injured closer Mariano Rivera, who plans to return in 2013 from a knee injury.
Soriano decided in October to decline a $14 million option for 2013, taking a $1.5 million buyout from the Yankees and entering free agency instead.
"When the opportunity with Washington came about, I thought it was a club that I could help now and obviously help win a World Series," Soriano said through an interpreter.
His contract with Washington is worth $14 million in both 2013 and 2014, with half the money each season deferred until $2 million payments are made each Jan. 5 from 2018-24. The deal also includes a $14 million club option for 2015, which becomes guaranteed if Soriano finishes 120 games in 2013 and 2014 combined. Half of the money for that option also would be deferred, increasing the annual 2018-24 payments to $3 million
Washington will lose its first-round draft pick in June's amateur draft, while the Yankees will gain an extra pick after the first round as compensation for losing Soriano.
"I'm a big believer of building through the draft, but I think a few things were different this year. ... Picking at the end of the draft, like we are this year, we examined the draft pool ... (and) it was a good time for us to forfeit the pick."
Rizzo also spoke about the three-team swap Wednesday in which Washington sent outfielder Michael Morse to Seattle and reacquired pitching prospect A.J. Cole, who the Nationals dealt to Oakland 13 months ago in the trade for Gio Gonzalez. Washington also got minor league pitcher Blake Treinen and a player to be named.
"The phone call with Mike was kind of bittersweet," Rizzo said. "He handled it terrifically. ... He told me he appreciated everything we've done for him. I told him we appreciate everything he's done for us. 'Go out, have a good year and make a lot of money."'