Herennius Etruscus (Latin: Quintus Herennius Etruscus Messius Decius Augustus; ca. 227 – June 251), was Roman emperor in 251, in a joint rule with his father Decius. Emperor Hostilian was his younger brother.

Herennius was born in near Sirmium in Pannonia (now Sremska Mitrovica, Serbia), during one of his father's military postings. His mother was Herennia Cupressenia Etruscilla, a Roman lady of an important senatorial family. Herennius was very close to his father and accompanied him in 248, as a military tribune, when Decius was appointed by Philip the Arab to deal with the revolt of Pacatianus in the Danube frontier. Decius was successful on defeating this usurper and felt confident to begin a rebellion of his own in the following year. Acclaimed emperor by his own troops, Decius marched into Italy and defeated Philip near modern Verona. In Rome, Herennius was declared heir to the throne and received the title of princeps iuventutis (prince of youth).