Tincommius or Tincomarus was king of the Iron Age tribe of the Atrebates who lived in southern central Britain immediately prior to the Roman invasion.

He was the son and heir of Commius and succeeded his father around 20-25 BC. Little is known of his reign although numismatic] evidence suggests that he was more sympathetic to Rome than his father was in later years: the coins he issued much more closely resemble Roman types, and are so much better made that they must have come from professional Roman die-cutters. GC Boon has suggested that this technical advance was not limited to coinage and represents wider industrial assistance from the Roman Empire. Tincommius' successors used the term rex on their coins and this indicates that Tincommius had begun the process of achieving client kingdom status with Rome.

By 16 BC Roman pottery and other imports appear in considerable quantities at Tincommius' capital of Calleva Atrebatum, today known as Silchester, and it is likely that the Atrebatic king had established trading and diplomatic links with Augustus.

Tincommius was expelled by his subjects for unknown reasons around AD 8 and fled to Rome as a refugee and supplicant. He was replaced by Eppillus whom Augustus chose to recognise as rex rather than depose and reinstate Tincommius. Augustus may have planned to use his ally's deposing as an excuse to invade Britain but other, more pressing foreign policy matters probably persuaded him to postpone the move.