Each deal is worth $2.5 million, plus $300,000 in bonuses, and Blues general manager Doug Armstrong said the franchise isn't necessarily renting the players.
"I just think they wanted to make sure they were going into a situation that they thought was best for them, not only this year but both players want to play multiple years," Armstrong said. "If we have success as a team and as a group, I don't see why these relationships can't continue."
The Blues have been busy on a minor scale in free agency, also signing veteran forward Scott Nichol and defenseman Kent Huskins. They re-signed forwards T.J. Oshie and Matt D'Agostini before free agency.
"We feel we've really improved our team since July 1st," Armstrong said. "I think we're very comfortable with the group that we have."
The 35-year-old Langenbrunner is coming off a disappointing season, totaling nine goals and 23 assists in 70 games with Dallas and New Jersey. In 2009-10, Langenbrunner had 61 points with the Devils.
The 36-year-old Arnott is a 17-year veteran and had 17 goals and 14 assists in 73 games last season for New Jersey and Washington. His best season was with Dallas in 2005-06 when he had 76 points.
Arnott was captain of the Nashville Predators for three seasons from 2007-10. He ranks sixth among active players in career games and seventh in points, and has 12 seasons with 20 or more goals.