Lucius Decidius Saxa (died 40 BC), sometimes mistaken as Decidus, was a Roman general in the 1st century BC.
He was born in Spain, but perhaps of Italian origin. In 49 BC he fought as follower of Julius Caesar in Spain against an army of Pompey. In 44 BC he was tribune of the people and after the assassination of the dictator he went over to Mark Antony. At the beginning of 43 BC Antony was besieged in Mutina and was assisted by Decidius Saxa. In 42 BC, after the founding of the second Triumvirate, Saxa was, together with Gaius Norbanus Flaccus, appointed by Mark Antony to lead the recognition force of eight legions into Thrace before the Battle of Philippi.
Saxa later went on to be appointed governor of Syria by Antony (41 BC) while Norbanus was elected consul in 38 BC, recognizing the great prestige of the victory over the liberatores. He was heavily defeated near Antioch, when Quintus Labienus led a Parthian intervention in Syria in 40 BC. He fled to Cilicia where he was captured and executed by the Parthians. His legions reportedly suffered heavy defeats and several of his aquilae were seized, being returned to Rome first after a brief Roman war against Parthia and negotiations by then Roman emperor Augustus in 20 BC. The eagle standards were returned together with those captured from Marcus Licinius Crassus in 53 BC, a great propaganda victory for Rome.
A brother of Decidius Saxa was his Quaestor in Syria in 40 BC and fought with him against the Parthians, but his soldiers deserted to the enemy so that he had to surrender to Quintus Labienus.