Rerum novarum is an encyclical issued by Pope Leo XIII on 15 May 1891. It is an open letter passed to all Catholic bishops addressing the condition of the working class. It discusses the relationships and mutual duties between labor and capital, as well as between government and its citizens. Of primary concern is the need to alleviate the poverty of the working class. The encyclical attempts to strike a just balance among the rights of workers, owners, and the state, supporting the rights of labor to form trade unions and earn a living wage, balanced with the right to private property, while rejecting the extremes of state socialism and laissez-faire capitalism.

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), Paul VI's Octogesima adveniens (1971), John Paul II's Centesimus annus (1991), and Leo XIV's Magnifica humanitas (2026), each commemorating an anniversary of Rerum novarum.

The encyclical also inspired Catholic activism and influenced the development of both distributism and corporatism. Socialists generally contest the encyclical's interpretation of socialism; Catholic socialists in particular, interpreting Rerum novarum as not rejecting socialism, argue that divine law justifies the abolition of private property and emphasize the encyclical's anti-capitalist character.

The first draft and content of the encyclical was written by Tommaso Maria Zigliara, professor from 1870 to 1879 at the College of Saint Thomas (rector after 1873), a member of seven Roman congregations including the Congregation for Studies, and co-founder of the Pontifical Academy of Saint Thomas Aquinas in 1879. Zigliara's fame as a scholar at the forefront of the Thomist revival was widespread in Rome and elsewhere. In addition to Rerum novarum, which elicited the strongest response in the United States, Zigliara had contributed to the drafting of the encyclical Aeterni Patris (1879), which addressed modern science and attempted to advance the revival of Scholastic philosophy; he was a strong opponent of traditionalism and ontologism, favoring instead the moderate philosophical realism of Thomas Aquinas, and was with Giuseppe Pecci one of the Thomist cardinals of Leo.

The encyclical was made possible by the writings of the precursors of economic personalism, in particular the Jesuit fathers Luigi Taparelli D'Azeglio and Matteo Liberatore. The latter was one of the authors of the document together with the Dominican Cardinal Tommaso Maria Zigliara. In drafting the encyclical, the Pope requested the collaboration of the then secretary for Latin letters Vincenzo Tarozzi. The German theologian Wilhelm Emmanuel von Ketteler and the British Cardinal Henry Edward Manning were also influential in its composition. Manning, who was a significant contributor to the development of the encyclical, encouraged English Catholics to engage in politics and seek economic justice, even going so far as to support the 1889 London dock strike.

Rerum novarum explicitly addresses the condition of the working class and is subtitled "On the Conditions of Labor", reflecting the need that a "remedy must be found quickly for the misery and wretchedness pressing so unjustly on the majority of the working class". In this encyclical, Leo articulates the Catholic Church's response to the social conflict in the wake of capitalism and industrialization, which had provoked socialist and communist movements and ideologies, as well as to the emerging economic liberal and Marxist theories. The encyclical defends the right of workers to form unions and the institution of property, as well as the right to a living wage. At the same time, it condemns socialism and capitalism, particularly state socialism and competitive or laissez-faire capitalism, while avoiding more radical alternatives like Georgism.

The Pope declares that the role of the state is to promote justice through the protection of rights, while the Church must speak out on social issues to teach correct social principles and ensure class harmony, calming class conflict. He restates the Church's long-standing teaching regarding the crucial importance of private property rights but recognizes, in one of the well-known passages of the encyclical, that the free operation of market forces must be tempered by moral considerations. In that passage, he writes: "Let the working man and the employer make free agreements, and in particular let them agree freely as to the wages; nevertheless, there underlies a dictate of natural justice more imperious and ancient than any bargain between man and man, namely, that wages ought not to be insufficient to support a frugal and well-behaved wage-earner. If through necessity or fear of a worse evil the workman accept harder conditions because an employer or contractor will afford him no better, he is made the victim of force and injustice."

Rerum novarum is remarkable for its vivid depiction of the plight of the 19th-century urban poor and for its condemnation of laissez-faire capitalism. Among the remedies it prescribes are the formation of trade unions and the introduction of collective bargaining, particularly as an alternative to state intervention. Although the encyclical follows traditional teaching concerning the rights and duties of property and the relations of employer and employee, it applies the old doctrines specifically to modern conditions, hence the title. Leo first quotes Aquinas in affirming that private property is a fundamental principle of natural law. He then quotes Gregory the Great regarding its proper use: "He that hath a talent, let him see that he hide it not; he that hath abundance, let him quicken himself to mercy and generosity; he that hath art and skill, let him do his best to share the use and the utility hereof with his neighbor." Rerum novarum also recognizes the special status of the poor in relation to social issues, expressing God's compassion and favor for them; this is elaborated in the modern Catholic principle of the "preferential option for the poor".