The four-year deal is worth $26.6 million with an annual salary-cap hit of $6.65 million, according to multiple reports.
"I think [the decision] says that I trust that the New York Rangers believe in me," Shattenkirk said on a conference call, according to the New York Post. "The comfortability factor was huge. Maybe money was left on the table, but when you have an opportunity to fulfill a lifelong dream like this that might come once in my career, this was my chance. A lot of factors other than money and term came into play, and that made the decision for me."
The Capitals acquired Shattenkirk in a trade-deadline deal with the Blues, sending forward Zach Sanford, a 2017 first-round draft pick and a conditional pick to St. Louis. Shattenkirk posted 14 points in 19 regular-season games for Washington but was less effective in the postseason, as the Presidents' Trophy-winning team was eliminated in the second round.
The right-handed defenseman with offensive pop set career highs in assists and points in 2016-17.
Shattenkirk is one of 14 defensemen in NHL history who registered 30 or more assists and 40 or more points in at least six of their first seven seasons. The only season in which Shattenkirk didn't reach those plateaus was 2012-13, a 48-game season.
Shattenkirk was a 2007 first-round pick by the Colorado Avalanche. He began his NHL career in 2010-11 before being dealt to the Blues in his rookie season. He has 68 goals and 230 assists in 490 career games.
The Associated Press contributed to this report.