Caius Messius Quintus Decius was a Roman senator from Sirmium (now Sremska Mitrovica) in Pannonia.
Prior to his accession in 249, he had a long cursus honorum well evidenced by inscriptions: he became governor of Moesia Inferior and Germania Inferior at the end of the reign of Severus Alexander, and was a supporter of Maximinus Thrax who appointed him legatus of Hispania Citerior. In 248/9, he was named by Philip I governor of the provinces of Moesia and Pannonia to restore state authority in this key area of the Empire after the usurpation of Pacatianus. Decius himself was proclaimed emperor by his troops in June 249; he then defeated and killed Philip I near Verona(?). Decius took up residence at Rome in late September 251. He was given the name Traianus by the Senate.
Since the outbreak of hostilities in 248, the emperors had been fully engaged in the war against the Goths. To reaffirm the Pax Deorum and to legitimize his position, Decius issued an Edict in winter 249/250 to oblige every citizen of the Roman Empire to sacrifice to the traditional pagan gods on behalf of Rome and the Imperial Family. This decision led to the first general persecution of Christians. Unfortunately, at the same time, there was an outbreak of plague which ravaged everything in its path.
The rest of this short reign was entirely devoted to the military campaigns on the Danubian frontier. An old and competent senator was appointed governor of Moesia (Trebonianus Gallus) and Decius and his elder son attacked and defeated the Carpians in Dacia in the winter of 250/1. In Moesia, the situation was critical since the Goths had plundered Thrace and important cities like Nicopolis ad Istrum and Philippopolis. In early 251, Decius and Etruscus carried out a new military campaign with the aim of driving out the Goths. The fighting was serious and uncertain. Etruscus probably died first during a skirmish in early spring 251. Decius suffered the same fate during the battle of Abritus in the plain of Dobrudja. Decius was the first Emperor to die in battle against barbarians.
Two usurpers named L.(?) Priscus (governor of Thrace) and Iulius Valens Licinianus are recorded in the last months of his reign.