Quadragesimo anno (Latin pronunciation: [kʷa.draˈd͡ʒɛː.si.mo ˈan.no]) (Latin for "In the 40th Year") is an encyclical issued by Pope Pius XI on 15 May 1931, 40 years after Leo XIII's encyclical Rerum novarum, further developing Catholic social teaching. Unlike Leo XIII, who addressed the condition of workers, Pius XI discusses the ethical implications of the social and economic order. He describes the major dangers for human freedom and dignity arising from unrestrained capitalism, socialism, and communism as practised in Russia. He also calls for the reconstruction of the social order based on the principles of solidarity and subsidiarity.

Essential contributors to the formulation of the encyclical were the German Jesuits, Roman Catholic theologians and social philosophers Gustav Gundlach and the Königswinter Circle through one of its main authors Oswald von Nell-Breuning.