The Blues get forward Zach Sanford, a 2017 first-round draft pick, and a conditional future second-round pick.
ESPN's Pierre LeBrun reports the conditional pick could be as high as a second-rounder or as low as a seventh-rounder, depending on factors such as whether Shattenkirk re-signs with Washington or is traded again before he hits free agency, or how far the Capitals progress in this year's postseason and how much Shattenkirk plays.
The Blues also retain 39 percent of the remaining portion of Shattenkirk's $5.2 million salary for this season, sources told ESPN.
"We felt it was important to acquire another defenseman to strengthen and add depth to our blue line,'' Capitals general manager Brian MacLellan said in a statement. "Kevin is a skilled, puck-moving defenseman who we think will help our team at even strength and on the power play.''
MacLelllan said Tuesday that Shattenkirk will arrive in New York later in the day and be in the lineup when the Capitals face the Rangers Tuesday night.
Washington foward T.J. Oshie, a former Blues player, told the St. Louis Post-Dispatch he "couldn't be more excited" about reuniting with Shattenkirk.
Shaaaaaattttini!!!!!!! Welcome to D.C. buddy. Excited to be back together!!! @shattdeuces
— TJ Oshie (@TJOshie77) February 28, 2017
Shattenkirk, 28, a right-handed defenseman with offensive pop, has 11 goals and 31 assists in 61 games this season for the Blues. He is on pace to surpass his career high of 45 points (10 goals, 35 assists) in 2013-14.
A 2007 first-round pick of the Colorado Avalanche, Shattenkirk was traded to the Blues during his rookie season in 2010-11.
The Blues and Lightning agreed to a deal for Shattenkirk last week that would have included a long-term extension with Tampa Bay, according to the St. Louis Post-Dispatch. But Shattenkirk, who has a no-trade clause, reportedly rejected that deal because he wants to test the free-agent market after the season.