Casimiro José Marques de Abreu (January 4, 1839 – October 18, 1860) was a Brazilian poet, novelist and playwright, adept of the "Ultra-Romanticism" movement.

He received only a basic education at Instituto Freeze, in Nova Friburgo, where he met and befriended Pedro Luís Pereira de Sousa.

Following orders of his father, he moved to Rio de Janeiro in 1852 to dedicate himself to commerce, an activity which he hated.

There he began his literary career, writing for many newspapers (such as O Progresso and Ilustração Luso-Brasileira) and collaborating with Alexandre Herculano and Luís Augusto Rebelo da Silva, among others.

Suffering from tuberculosis, Casimiro moved to Nova Friburgo in order to recover, but he died at age 21 on October 18, 1860.