Financial terms of the contracts were not revealed, but TSN's Darren Dreger reported Arnott's at less than $3 million.
"Jason is a veteran forward and a Stanley Cup champion who will be a valuable addition to our club. He's a proven leader and we are excited that he has decided to wear the Bluenote," Armstrong said.
On Langenbrunner, he added: "We're excited to add a veteran forward with two Stanley Cup rings to our roster. Jamie is a solid player who will bring years of NHL experience on and off the ice for our hockey club."
Langenbrunner and Arnott, who have combined for 32 NHL seasons and three Stanley Cups, join a team that averaged 27 years of age. In fact, just three players were over 30 prior to the signings -- forward Andy McDonald (33), and defensemen Kent Huskins (32) and Barret Jackman (30). Huskins signed a one-year contract with the Blues on Saturday.
Arnott, 36, had 13 goals and 11 assists in 62 games with the New Jersey Devils, and then was traded to the Washington Capitals on Feb. 28 for Dave Steckel and a 2012 second-round pick. He had 4 goals and 3 assists in 11 games with the Caps, and then had 6 points in nine playoff games.
He has 400 goals and 504 assists in 1,172 games with Oilers, Devils, Stars, Predators and Capitals. He scored the game-winning double-overtime goal to win the Stanley Cup for the Devils in 2000.
Langenbrunner, 35, won Stanley Cups with Dallas in 1999 and New Jersey in 2003. He had 4 goals and 10 assists in 31 games for the Devils when he was traded to the Dallas Stars on Jan. 7 for a 2011 third-round pick that the Devils turned into Indiana Ice (USHL) center Blake Coleman. Langenbrunner had 5 goals and 13 assists in 39 games with the Stars.
Follow Mike Morreale on Twitter at: @mike_morreale
I've been getting frustrated lately, and the only thing keeping me sane was the team winning and other people stepping up and scoring. Then you just kind of let it go and realize you can end the series with one shot, that frustration goes away for a brief moment, and that's what happened.
— Montreal forward Max Pacioretty after scoring the series winner in Game 4 -- his first career playoff goal -- to eliminate the Lightning and send the Canadiens into the second round