Théophile-Jules-Henri "Theo" Marzials (20 December 1850 – 2 February 1920) was a British composer, singer and poet. Marzials was described in 1894 as a "poet and eccentric" by parodist Max Beerbohm, and, after writing and performing several popular songs, vanished into obscurity. His poetry is seen as an example of 19th-century aestheticism.
Marzials's father, Antoine-Theophile Marzials, was the pastor of the French Protestant Church of London and had been a clergyman before visiting London in 1839. While there he met Mary Ann Jackson (Marzials' mother) and the couple married. Theo was the youngest of their five children. He was educated at Merchant Taylors' School His brother, Frank Marzials, was a prolific author of poetry, essays, and biographies and an accountant general in the army; he was knighted in 1904.
In 1870 Marzials started work at the British Museum as a junior assistant in the librarian's office. There he would work with Coventry Patmore, John Payne, Arthur O'Shaughnessy, and Edmund Gosse, with whom he would form a particularly close friendship.
In 1873 Marzials's only published collection of poetry was released, The Gallery of Pigeons and Other Poems, which included the anthologized love poem "A Fragment" ("And then it seem'd I was a bird...") It also includes A Tragedy, an unusual poem that has often been called the worst ever written in the English language. The poem has been chosen as the worst ever by Ross and Kathryn Petras in the 1997 book Very Bad Poetry and by the Not Terribly Good Club of Great Britain along with many other writers and critics. However, Ford Madox Brown called the collection "by far the most exquisite ... by any of the lesser Pre-Raphaelite poets". Other poems by Marzials featured in The Yellow Book, an important literary periodical of the late 19th century.Gerard Manley Hopkins described Marzials's "Rondel" as having "an art and finish rare in English verse".