Joseph Leo Cardijn (Dutch: [ˈjoːzəf ˈleːjoː kɑrˈdɛin]; 13 November 1882 – 24 July 1967) was a Belgian Catholic cardinal and the founder of the movement of Young Christian Workers (Jeunesse ouvrière chrétienne, JOC).

Cardijn was best known for his lifelong dedication to social activism and working towards the improvement of the working class; after his ordination, he made a particular focus of his life the effort to evangelize and bring the core messages of faith in the Gospel back to the working class, who he believed were neglected. He was not wrong in that assessment since old schoolmates working in the mines and mills believed the Church had abandoned them, which prompted Cardijn to found a social movement dedicated to this task despite the opposition that it faced.

His movement received vocal praise and encouragement from Pope Pius XI. He demonstrated great zeal even after he had been imprisoned during World War II, which served to empower his positions on social change and to oppose rising inequalities. Cardijn was also an extensive traveller (visiting places such as Costa Rica and Australia) and was a recipient of several recognitions for his work in social activism. He served as a critical contributor to the Second Vatican Council, working closely with Pope John XXIII and Pope Paul VI. The latter named Cardijn as a cardinal in 1965.

Cardijn died in a Leuven hospital in 1967. The cause for the cardinal's beatification was initiated in 2014 and he is titled as a Servant of God.

Joseph Cardijn was born on 13 November 1882 in Schaerbeek as the second of four children to Henri Hieronimus Cardijn (1840–1903) and Louise Maria Ester Van Daelen (c. 1850–1923). His siblings were: Jeanne (1881–1912), Victor (1884–1908), and Charles (1888–1975). He lived with his grandparents in Halle until his parents quit their jobs as caretakers and his father (who was illiterate) started a business as a coal merchant while his mother opened a café. But he also lived with his grandparents because his mother's frail health meant she could not nurse him. He received his First Communion in 1895. His mother would often gather her children to read scriptural stories to them such as Creation or Pentecost or fairy tales such as Little Red Riding Hood.

Both parents were devout and agreed to let him continue his studies to become a priest instead of going to work. One night in 1895 he went to his shared bedroom with his siblings and then crept downstairs barefoot to the kitchen to the fireside to speak with his parents regarding his desire to become a priest. He pleaded with his father, who relented after two tears fell down his cheeks; Cardijn affirmed: "I've felt within me a call from God. I want to be a priest". He commenced his ecclesial education in 1897. His parents had wanted him to have a good job since it would add to their income and ensure the Cardijns were better off.

He studied in Halle before going in September 1903 to Mechelen for his education; his father died a few months prior to this. During his vacations he visited his old schoolmates working in the mills and the mines. But his old friends scoffed at his desire to become a priest, believing he joined those that oppressed the working class.

Cardijn was ordained a priest in St. Rumbold's Cathedral on 22 September 1906 by Cardinal Désiré-Joseph Mercier. It was around this time that fellow priest Adolf Daens befriended him, becoming a major influence on Cardijn's life. He had also decided to spend his life evangelizing and bringing the faith back to the working class, who he felt had been neglected.