Hostilian (Latin: Gaius Valens Hostilianus Messius Quintus Augustus; 230? – 251) was Roman emperor in 251. Hostilian was born in Sirmium (now Sremska Mitrovica, Serbia) in Illyricum sometime after 230, as the son of the future emperor Decius by his wife Herennia Cupressenia Etruscilla. He was the younger brother of emperor Herennius Etruscus.

Following his father's accession to the throne, Hostilian received the treatment of an imperial prince, but was always kept in the shade of his brother Herennius, who enjoyed the privileges of being older and heir. In the beginning of 251, Decius elevated his son Herennius to co-emperor and Hostilian succeeded him in the title of princeps iuventutis (prince of youth). Decius and Herennius then set out on campaign against king Cniva of the Goths, to punish him for raids on the Danubian frontier. Hostilian remained in Rome due to his inexperience, and empress Herennia was named regent.

The campaign proved to be a disaster: both Herennius and Decius died in the Battle of Abrittus and became the first two emperors to be killed by a foreign army in battle. The armies in the Danube acclaimed Trebonianus Gallus emperor, but Rome acknowledged Hostilian's rights. Since Trebonianus was a respected general, there was fear of another civil war of succession, despite the fact that he chose to respect the will of Rome and adopted Hostilian. But later in 251, the Plague of Cyprian broke out in the Empire and Hostilian died in the epidemic. He was the first emperor in 40 years to die of natural causes, one of only 13. His death opened the way for the rule of Trebonianus with his natural son Volusianus.