Marquis will receive $7.5 million in each of the next two seasons under Tuesday's deal.
The 31-year-old Marquis is a right-hander who went 15-13 with a 4.04 ERA and 115 strikeouts in 216 innings for the Colorado Rockies in 2009. He made the NL All-Star team last season, thanks to an 11-6 record and 3.65 ERA at that point. But Marquis fizzled down the stretch and wasn't part of the Rockies' postseason starting rotation.
Still, he brings the sort of veteran, innings-eating presence that Washington's starting rotation has been lacking.
Marquis made at least 32 starts in five of the past six seasons; the lone exception was 2008, when he started 28 games for the Cubs.
Only one pitcher, left-hander John Lannan, made 20 or more starts for Washington last season, when the club finished with the worst record in the majors, 59-103. It was the second consecutive year that the Nationals lost more than 100 games.
The addition of Marquis represents a building block and a step toward fixing one of the Nationals' most glaring weaknesses: starting pitching. He's also the club's biggest move of the offseason so far, following the signing of free-agent catcher Ivan Rodriguez to a $6 million, two-year deal, and a trade with the New York Yankees for relief pitcher Brian Bruney.
Marquis, who went to high school on Staten Island, had been interested in signing with a New York team. Yankees general manager Brian Cashman said the pitcher was brought up when he traveled to the winter meetings with Marquis' agents, Sam and Seth Levinson.
Because Marquis was a Type B free agent and was offered arbitration by the Rockies, Colorado stands to receive a compensatory pick between the first and second rounds in June's amateur draft.