The Roselle Catholic High School Lions compete in the Union County Interscholastic Athletic Conference, which includes public and private high schools in Union County and operates under the supervision of the New Jersey State Interscholastic Athletic Association (NJSIAA) and was established as part of a reorganization of sports leagues in Northern New Jersey. Prior to the NJSIAA's 2009 realignment, the school had participated in the Mountain Valley Conference, which included public and private high schools in Essex County, Somerset County and Union County. With 255 students in grades 10-12, the school was classified by the NJSIAA for the 2019–20 school year as Non-Public B for most athletic competition purposes, which included schools with an enrollment of 37 to 366 students in that grade range (equivalent to Group II for public schools).