Cosmas (died 727) was an unsuccessful Byzantine usurper during the reign of Leo III.

In 726/727, a revolt broke out in Hellas against the Iconoclast policies of Leo III. Cosmas was declared emperor by the rebels. The insurrection was soon crushed in April 726/7 and Cosmas was beheaded.

See also Basil Onomagoulos Tiberius Petasius

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