Salvador, who can become an unrestricted free agent after the season, gives a blue line boost to the Devils. New Jersey has scratched veteran defensemen Karel Rachunek and Vitaly Vishnevski in recent games.
The deal was completed a few hours before the NHL trade deadline.
"In Bryce Salvador, we have acquired a veteran NHL defenseman with good size and strength who plays a physical game," Devils general manager Lou Lamoriello said. "We look forward to the depth Bryce will bring to our roster."
Salvador, 32, is in his seventh NHL season -- all with St. Louis -- and is earning $1.5 million. In 56 games this season, Salvador has one goal and 10 assists.
Janssen, 23, is under contract for this season and next at $550,000 per year. In 95 career games with the Devils, the St. Louis native has one goal and 205 penalty minutes.
Janssen spent most of the season on the long-term injury list after injuring a shoulder during the preseason. Janssen played in three games with Lowell (AHL) on a conditioning stint before being activated by New Jersey on Monday night, presumably to be traded.
"We feel that Cam's style of play will bring energy and grit to our lineup, being from St. Louis, feel that he will be an instant fan favorite here," Blues president John Davidson said. "Our depth at defense enabled us to make this move."