Thomas the Slav (c. 760 – October 823) was a 9th century Byzantine military commander, most notable for leading a wide-scale revolt in 821–23 against Emperor Michael II the Amorian (ruled 820–29).

An army officer of Slavic origin from the Pontus region (now northeastern Turkey), Thomas rises to prominence, along with the future emperors Michael II and Leo V the Armenian (r. 813–820), under the protection of general Bardanes Tourkos.

After Bardanes's failed rebellion in 803, Thomas falls into obscurity until Leo V's rise to the throne, when Thomas is raised to a senior military command in central Asia Minor.

After the murder of Leo and usurpation of the throne by Michael the Amorian, Thomas revolts, claiming the throne for himself.

Thomas quickly secures support from most of the themes (provinces) and troops in Asia Minor, and concludes an alliance with the Abbasid Caliphate.

After winning over the maritime themes and their ships, he crosses with his army to Europe and lays siege to Constantinople.

Michael II calls for help from the Bulgarian ruler Omurtag, whose troops attack Thomas's army.

Although repelled, the Bulgarians inflict heavy casualties on Thomas's men, who break and flee when Michael takes to the field a few months later.

Thomas seeks refuge in Arcadiopolis, where he is soon seized by Michael's troops and executed.

Thomas's rebellion is one of the largest in the Byzantine Empire's history, but its precise circumstances are unclear due to competing historical narratives, including claims fabricated by Michael that have distorted accounts of the revolt.

Consequently, various motives and driving forces have been attributed to it.

Its effects on the military position of the empire are also disputed.