Financial terms of the deal were not revealed.
The 27-year-old Janssen spent the last three-plus seasons with the St. Louis Blues after being traded by the Devils in exchange for defenseman Bryce Salvador on Feb. 26, 2008. Janssen had 1 goal, 3 assists and 131 penalty minutes in 54 games with the Blues in 2010-11.
In 95 games with the Devils spanning two seasons (2005-06 and 2006-07), Janssen had 1 goal and 205 penalty minutes.
Janssen's agent, Kevin Magnuson, confirmed to Tom Gulitti of The Bergen Record that the deal with the Devils came not long after the Devils traded Pierre-Luc Letourneau-Leblond to Calgary for a 2012 fifth-round draft choice on Thursday afternoon.
"(Cam) is very excited to be back with Lou (Lamoriello) and the Devils," Magnuson told Gulitti. "It's a great opportunity for Cam with Pierre being traded earlier in the day."