Basil Onomagoulos (Greek: Βασίλειος Ὀνομάγουλος) was a Byzantine official who was declared rival emperor in Sicily in 717, taking the regnal name Tiberius.
In 717 he was a member of the entourage of Sergius, the Byzantine governor of Sicily, when news arrived to the island of the fall of Constantinople, the Byzantine imperial capital, to an Arab siege.
At this point Sergius declared Basil as emperor, taking the name Tiberius.
The Emperor Leo III the Isaurian (r. 717–741) however quickly dispatched a chartoularios named Paul, whom he named patrikios and strategos of Sicily, with a few men and imperial instructions for the army.
Sergius himself managed to escape to the Lombards in mainland Italy, and returned only after he received guarantees that no harm would befall him.