Rerum novarum (from its first two words, Latin for "of revolutionary change"), or Rights and Duties of Capital and Labor, is an encyclical issued by Pope Leo XIII on 15 May 1891. It was an open letter, passed to all Catholic bishops, that addressed the condition of the working classes.

It discussed the relationships and mutual duties between labor and capital, as well as government and its citizens. Of primary concern was the need for some amelioration of "The misery and wretchedness pressing so unjustly on the majority of the working class." It supported the rights of labor to form unions, rejected socialism and unrestricted capitalism, whilst affirming the right to private property.

"Rerum Novarum" is considered a foundational text of modern Catholic social teaching. Many of the positions in Rerum novarum were supplemented by later encyclicals, in particular Pius XI's Quadragesimo anno (1931), John XXIII's Mater et magistra (1961), and John Paul II's Centesimus annus (1991).