Gaius Valerius Flaccus (late 1st century AD) was a Roman poet, who flourished under the emperors Vespasian and Titus.

He has been identified on insufficient grounds with a poet friend of Martial (i. 61. 76), a native of Padua, and in needy circumstances; but as he was a member of the College of Fifteen, who had charge of the Sibylline books (i. 5), he must have been well off. The subscription of the Vatican manuscript, which adds the name Setinus Balbus, points to his having been a native of Setia in Latium. The only ancient writer who mentions him is Quintilian (Instil. Orat. x. I. 90), who laments his recent death as a great loss; as Quintilian's work was finished about AD 90, this gives a limit for the death of Flaccus.

His major work, the Argonautica, dedicated to Vespasian on his setting out for Britain, was written during the siege, or shortly after the capture, of Jerusalem by Titus (70). As the eruption of Vesuvius (79) is alluded to, it must have occupied him a long time. The Argonautica is an epic in eight books on the Quest for the Golden Fleece. The poem is in a very corrupt state, and ends abruptly with the request of Medea to accompany Jason on his homeward voyage. It is a disputed question whether part has been lost or whether it was ever finished. It is a free imitation and in parts a translation of the work of Apollonius of Rhodes, already familiar to the Romans in the popular version of Varro Atacinus. The object of the work has been described as the glorification of Vespasian's achievements in securing Roman rule in Britain and opening up the ocean to navigation (as the Euxine was opened up by the Argo).

Various estimates have been formed of the genius of Flaccus, and some critics have ranked him above his original, to whom he certainly is superior in liveliness of description and delineation of character. His diction is pure, his style correct, his versification smooth though monotonous. On the other hand, he is wholly without originality, and his poetry, though free from glaring defects, is artificial and elaborately dull. His model in language was Virgil, to whom he is far inferior in taste and lucidity. His tiresome display of learning, rhetorical exaggeration and ornamentations make him difficult to read, which no doubt accounts for his unpopularity in ancient times.