Flavius Julius Constans (c. 323 – 350), also called Constans I, was Roman emperor from 337 to 350. He held the imperial rank of caesar from 333, and was the youngest son of Constantine the Great.

After his father's death, he was made augustus alongside his brothers in September 337. Constans was given the administration of the praetorian prefectures of Italy, Illyricum, and Africa. He defeated the Sarmatians in a campaign shortly afterwards. Quarrels over the sharing of power led to a civil war with his eldest brother and co-emperor Constantine II, who invaded Italy in 340 and was killed in battle by Constans's forces near Aquileia. Constans gained from him the praetorian prefecture of Gaul. Thereafter there were tensions with his remaining brother and co-augustus Constantius II (r. 337–361), including over the exiled bishop Athanasius of Alexandria, who in turn eulogized Constans as "the most pious Augustus... of blessed and everlasting memory." In the following years he campaigned against the Franks, and in 343 he visited Roman Britain, the last legitimate emperor to do so until Manuel II in 1400, more than a thousand years later.

In January 350, Magnentius (r. 350–353) the commander of the Jovians and Herculians, a corps in the Roman army, was acclaimed augustus at Augustodunum (Autun) with the support of Marcellinus, the comes rei privatae. Magnentius overthrew and killed Constans. Surviving sources, possibly influenced by the propaganda of Magnentius's faction, accuse Constans of misrule and of homosexuality.

Sources variously report Constans' age at the time of his death as 27 or 30, meaning he was born in either 320 or 323. Timothy Barnes, observing numismatic evidence, considered the younger age to be more likely. He was the third and youngest son of Constantine I and Fausta. According to the works of both Ausonius and Libanius, he was educated at Constantinople under the tutelage of the poet Aemilius Magnus Arborius, who instructed him in Latin.

On 25 December 333, Constans was elevated to the imperial rank of caesar at Constantinople by his father. Prior to 337, Constans became engaged to Olympias, the daughter of the praetorian prefect Ablabius, although the two never actually married.

After Constantine's death, Constans and his two brothers, Constantine II and Constantius II were proclaimed augusti and divided the Roman empire among themselves on 9 September 337. Constans was left with Italy, Africa and Illyricum. In 338, he campaigned against the Sarmatians.

Meanwhile, Constans came into conflict with his eldest brother Constantine II over the latter's presumed authority over Constans' territory. After attempting to issue legislation to Africa in 339, which was part of Constans' realm, Constantine led his army into an invasion of Italy only a year later. However, he was ambushed and killed by Constans' troops, and Constans then took control of his brother's territories.

Constans began his reign in an energetic fashion. From 341 to 342, he led a campaign against the Franks where, after an initial setback, the military operation concluded with a victory and a favorable peace treaty. Eutropius wrote that he "had performed many gallant actions in the field, and had made himself feared by the army through the whole course of his life, though without exercising any extraordinary severity," while Ammianus Marcellinus remarked that Julian was the only person the Alamanni feared after the death of Constans.