Quintus Herennius Etruscus Messius Decius, known simply as Herennius Etruscus, was briefly Roman emperor in 251, ruling jointly under his father Decius. His father was proclaimed emperor by his troops in September 249 while in Pannonia and Moesia, in opposition to Philip. Decius defeated Philip in battle, and was then proclaimed emperor by the Senate. Etruscus, still a child, was elevated to Caesar (heir) in 250, then further raised to Augustus (emperor) in May 251. When the Goths, under Cniva, invaded the Danubian provinces, he was sent with a vanguard, followed by the main body of Roman troops, led by Decius. They ambushed Cniva at the Battle of Nicopolis ad Istrum in 250, routing him, before being ambushed and routed themselves at the Battle of Beroe. Etruscus was killed in the Battle of Abritus the following year, alongside his father. After the deaths of both emperors, Trebonianus Gallus, who had been governor of Moesia, was elected emperor by the remaining Roman forces.

Gaius Messius Quintus Trajanus Decius, known as Trajan Decius or simply Decius, was Roman emperor from 249 to 251.