The Waldensians, also known as Waldenses (), Vallenses, Valdesi, or Vaudois, are adherents of a church tradition that began as an ascetic movement within Western Christianity before the Reformation. Originally known as the "Poor Men of Lyon" in the late twelfth century, the movement spread to the Cottian Alps in what are today France and Italy. The founding of the Waldensians is attributed to Peter Waldo, a wealthy merchant who gave away his property around 1173, preaching apostolic poverty as the way to perfection.

Waldensian teachings came into conflict with the Catholic Church and by 1215 the Waldensians were declared heretical, not because they preached apostolic poverty, which the Franciscans also preached, but because they were not willing to recognize the prerogatives of local bishops over the content of their preaching, nor to recognize standards about who was fit to preach. Pope Innocent III offered the Waldensians the chance to return to the Church, and many did, taking the name "Poor Catholics". Many did not, and were subjected to intense persecution and were confronted with organised and general discrimination in the following centuries. In the 16th century, the Waldensians were absorbed into the Protestant movement, under the influence of early Swiss reformer Heinrich Bullinger.

In some aspects the Waldensians of the Middle Ages could be seen as proto-Protestants, but they mostly did not raise the doctrinal objections characteristic of 16th century Protestant leaders. They came to align themselves with Protestantism: with the Resolutions of Chanforan on 12 September 1532, they formally became a part of the Calvinist tradition. They are members of the Community of Protestant Churches in Europe and its affiliates worldwide. They were nearly annihilated in the 17th century.

The main denomination within the movement was the Waldensian Evangelical Church, the original church in Italy. In 1975, it merged with the Methodist Evangelical Church to form the Union of Methodist and Waldensian Churches—a majority Waldensian church, with a minority of Methodists. Another large congregation is the Evangelical Waldensian Church of Río de la Plata in Argentina, Paraguay, and Uruguay.

Congregations continue to be active in Europe (particularly in the Piedmont region of Northern Italy), South America, and North America. Organizations, such as the American Waldensian Society, maintain the history of the movement and declare their mission as "proclaiming the Christian Gospel, serving the marginalized, promoting social justice, fostering inter-religious work, and advocating respect for religious diversity and freedom of conscience."