Rerum novarum (from its incipit, with the direct translation of the Latin meaning "of revolutionary change"), or Rights and Duties of Capital and Labor, is an encyclical issued by Pope Leo XIII on 15 May 1891. It is an open letter, passed to all Catholic patriarchs, primates, archbishops and bishops, that addressed the condition of the working classes.

It discusses the relationships and mutual duties between labor and capital, as well as government and its citizens. Of primary concern is the need for some amelioration of "the misery and wretchedness pressing so unjustly on the majority of the working class". It supports the rights of labor to form unions, rejects both socialism and unrestricted capitalism, while 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 are 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), each of which commemorates an anniversary of the publication of Rerum novarum.