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). The boys track team won the indoor track Non-Public state championship in 1965, 1967 and 1969, and won the Non-Public B title in 2011. The girls team won the Group II title in 2004 (as co-champion) and the Non-Public B title in 2009. The boys track team won the Non-Public indoor relay championships in 1967–1969, 1973 and 1974. The program's five state titles are tied for ninth-most in the state. The boys baseball team won the Non-Public A North state championship in 1967 and 1968, and won the Non-Public A state title in 1978 vs.
Camden Catholic High School. The 1978 team won the Parochial A state title with a 3–1 win against Camden Catholic in the championship game played at
Mercer County Park. The boys track team won the Non-Public Group A spring / outdoor track state championship in 1967 and 1973. The boys cross country running team won the Non-Public Group A state championship in 1968 and 1969, and won the Non-Public B title 2004 and 2008. The boys bowling team won the Group I state championship in 2008 and 2009. The girls volleyball team won the Non-Public state championship in 2006, defeating runner-up
Immaculate Heart Academy in the final match of the tournament. The boys' basketball team won the Non-Public B state championships in 2013-2015 (defeating
St. Anthony High School in the finals each of the three years) and 2018 (vs.
Ranney School). The team won their third consecutive Non-Public B title in 2015 with a 56–52 win against St. Anthony in the championship game. The team won the 2018 Non-Public B title with a 63–61 win against Ranney School on a basket scored with just over six seconds remaining in the championship game. The team came into the 2013
Tournament of Champions as the top seed and finished the season 25-5 after winning the program's first ToC title with a 78–54 win against fifth-seeded
Newark Tech High School in the semifinals and 65–49 against number-two seed
St. Joseph High School of Metuchen in the championship game. The 2015 team won the school's second ToC title with a 103–34 win against
Paulsboro High School in the semifinals and 57–45 against
Pope John XXIII Regional High School in the finals, behind 27 points by
Isaiah Briscoe. The team came into the 2018 ToC as the top seed and won the program's third title with a 75–62 win against
Nottingham High School in the semifinals and a 61–54 win against
Don Bosco Preparatory High School in the finals at the
CURE Insurance Arena in
Trenton, despite a career-high 33 points from Don Bosco's
Ron Harper Jr. ==Labyrinth==