Alberto Caeiro is a heteronym which the Portuguese poet Fernando Pessoa began to use in 1914 and introduced in print in 1925. In his fictional biography, Caeiro was born in Lisbon on 16 April 1889, lived most his life in a village in Ribatejo and died in 1915. He was the leader and teacher of a group of neopagan poets and intellectuals that included Pessoa's other heteronyms António Mora, Ricardo Reis and Álvaro de Campos.

The literary concept of the heteronym refers to one or more imaginary character(s) created by a writer to write in different styles. Heteronyms differ from pen names in that the latter are just false names, while the former are characters that have their own supposed physiques, biographies, and writing styles.