Ambrosius Aurelianus (Welsh: Emrys Wledig; Anglicised as Ambrose Aurelian and called Aurelius Ambrosius in the Historia Regum Britanniae and elsewhere) was a war leader of the Romano-British who won an important battle against the Anglo-Saxons in the 5th century, according to Gildas. He also appeared independently in the legends of the Britons, beginning with the 9th-century Historia Brittonum. Eventually, he was transformed by Geoffrey of Monmouth into the uncle of King Arthur, the brother of Arthur's father Uther Pendragon, as a ruler who precedes and predeceases them both. He also appears as a young prophet who meets the tyrant Vortigern; in this guise, he was later transformed into the wizard Merlin.

Ambrosius Aurelianus is one of the few people whom Gildas identifies by name in his sermon De Excidio et Conquestu Britanniae, and the only one named from the 5th century. De Excidio is considered the oldest extant British document about the so-called Arthurian period of Sub-Roman Britain. Following the destructive assault of the Saxons, the survivors gather together under the leadership of Ambrosius, who is described as:

a gentleman who, perhaps alone of the Romans, had survived the shock of this notable storm. Certainly his parents, who had worn the purple, were slain in it. His descendants in our day have become greatly inferior to their grandfather's [avita] excellence. Under him our people regained their strength, and challenged the victors to battle. The Lord assented, and the battle went their way.

Ambrosius was possibly of high birth and very likely a Christian: Gildas says that he won his battles "with God's help". Ambrosius's parents were slain by the Saxons and he was amongst the few survivors of their initial invasion.

According to Gildas, Ambrosius organised the survivors into an armed force and achieved the first military victory over the Saxon invaders. However, this victory was not decisive: "Sometimes the Saxons and sometimes the citizens [meaning the Romano-British inhabitants] were victorious." Due to Gildas's description of him, Ambrosius is one of the figures called the Last of the Romans.

Two points in Gildas's description have attracted much scholarly commentary. The first is what Gildas meant by saying Ambrosius' family "had worn the purple". Roman emperors and male Patricians wore clothes with a purple band to denote their class so the reference to purple may be to an aristocratic heritage. Roman military tribunes (tribuni militum), senior officers in Roman legions, wore a similar purple band so the reference may be to a family background of military leadership. The tradition was old, as the togas and pallia of already ancient senators and tribunes were trimmed with the purple band. In the church, "the purple" is a euphemism for blood and therefore "wearing the purple" may be a reference to martyrdom or a bishop's robe. In addition, in the later Roman Empire both Roman consuls and governors of consular rank also wore clothes with a purple fringe. The Notitia Dignitatum, a Roman catalogue of official posts, lists four or five provincial governors in Roman Britain and two of them were of consular rank. One was the governor of Maxima Caesariensis and the other that of Valentia. The parent who wore the purple may well have been one of these governors, whose names were not recorded.

It has been suggested by historian Alex Woolf that Ambrosius may have been related to the 5th-century Romano-British usurpers Marcus or Gratian – Woolf expresses a preference based on nomenclature for Marcus. Frank D. Reno, an Arthurian scholar, has instead argued that the name "Aurelianus" indicates the descent of Ambrosius from the Illyrian Roman emperor Aurelian (reigned 270–275). Aurelian's military campaigns included the conquest of the Gallic Empire. N. J. Higham suggests that Ambrosius may have been distantly related to imperial families of the late Roman Empire, such as the Theodosian dynasty. Branches of this particular dynasty were known to be active in western Roman provinces like Hispania.

Mike Ashley instead focuses on the name "Ambrosius" and its possible connection to Saint Ambrosius, a fourth-century Bishop of Milan, who also served as consular governor in areas of Roman Italy. The father of the Bishop is sometimes claimed to be a fourth century Praetorian prefect of Gaul named Aurelius Ambrosius, whose areas included Britain, though some modern scholars doubt that Saint Ambrosius was related to this man (instead identifying his father with an official named Uranius mentioned in an extract from the Theodosian Code). Ashley suggests that Ambrosius Aurelianus was related to the two Aurelii Ambrosii. Tim Venning points out that the name "Aurelianus" could be the result of a Roman adoption. When a boy was adopted into a new gens (clan), he received the family names of his new family but was often additionally called by a cognomen indicating his descent from his original family. The additional cognomen often had the form "-anus". When Gaius Octavius from gens Octavia was adopted by his uncle Gaius Julius Caesar, he was often distinguished from his adoptive father by the addition "Octavianus". In this case, Ambrosius may have been a member of gens Aurelia who was adopted by another gens/family.