Wellman, 24, has played in 41 NHL games -- 14 last season -- all with the Minnesota Wild, totaling 13 points. He was traded to New York on Feb. 3 in exchange for forward Erik Christensen and a seventh-round pick in the 2013 NHL Draft.
"Casey is a young, highly skilled forward who adds more depth to our team," Panthers GM Dale Tallon said in a statement. "He can play both center and right wing and we feel he has the ability to play in a top line role within our team. We look forward to his contributions to our club."
The Brentwood, Calif., native also played for Houston and Connecticut of the American Hockey League last season, scoring 23 goals with 24 assists in 57 games. He has 82 points in 99 career AHL games.
He was signed by the Wild as an undrafted free agent on March 16, 2010.