The Islanders also sent forward Claude Lapointe to Philadelphia for a fifth-round pick.
The 27-year-old Robitaille has five goals and 12 assists in 41 games for the Penguins. Last season he had 10 goals and 20 assists in 40 games.
"Randy is a gifted offensive player that we believe is a good fit for us," Islander General Manager Mike Milbury said.
Lapointe, 34, had six goals and six assists in 66 games for the Islanders. In all, he has scored 120 goals and has 173 assists in 823 games with four teams.
Flyer General Manager Bob Clarke said the move will strengthen the Flyers' penalty-killing unit and add depth to the bench.
Wesley, a 16-year veteran and the anchor of Carolina's defense for nine seasons, goes from a team well out of the playoffs to a Stanley Cup contender.
The 34-year-old Wesley has one goal, seven assists and 40 penalty minutes in 63 games this season.
Wesley played in 629 games for the Hurricanes -- third-most in franchise history -- after being acquired from the Boston Bruins in August 1994.