Mariana Vaz Alcoforado (Santa Maria da Feira, Beja, 22 April 1640 - Beja, 28 July 1723), was a Portuguese nun, living in the convent of the Poor Ladies (Convento de Nossa Senhora da Conceição) in Beja, Portugal.

Debate continues as to whether Mariana was the real Portuguese author of the Letters of a Portuguese Nun (comprising five letters). Her purported love affair with the French officer Noël Bouton, Marquis de Chamilly and later Marshall of France, has made Beja famous in (mainly Portuguese and French) literary circles.

Some literary scholars consider the letters a fictional work and their authorship is ascribed to Gabriel-Joseph de La Vergne, comte de Guilleragues (1628–1685), although a real nun named Mariana Alcoforado did exist. In her recent book Letters of a Portuguese Nun: Uncovering the Mystery Behind a Seventeenth-Century Forbidden Love (2006), the author Myriam Cyr has attempted to reassert the attribution of the letters to the real Mariana Alcoforado.