The theta nigrum (lit. 'black theta') or theta infelix (lit. 'unlucky theta') is a symbol of death in Greek and Latin epigraphy.^[1] Isidore of Seville notes the letter was appended after the name of a deceased soldier and finds of papyri containing military records have confirmed this use.^[1] Additionally it can be seen in the Gladiator Mosaic.

The term theta nigrum was coined by Theodor Mommsen. It consists of a circle with a diagonal line. The theta signified Thanatos, the Greek deity of death.^[2]