Other names: Ambrosius Aurelius, Ambrosius, Ambrosius Aurelianus, Ambr Emrys (Welsh)

According to Geoffrey of Monmouth, Aurelius Ambrosius was King Arthur's uncle, brother to Uther Pendragon. Aurelius and Uther are the children of the rightful king of Britain, living in exile in Brittany. They return to Britain as adults to take the throne from the usurper, Vortigern and to save the realm from the marauding Saxons that Vortigern allowed into Britain. Victorious in both of these endeavors, Aurelius orders Merlin and Uther to go to Ireland and bring back Stonehenge. Aurelius is later poisoned by a Saxon named Eopa on the orders of Vortigern's son, Paschent.

Aurelius almost certainly was a real person, and some have theorized that he was the historical basis for the legend of King Arthur, as a Briton who successfully made war against the Saxons. Gildas mentions Ambrosius Aurelianus by name, claiming that he was Roman (which may have been a political designation as much anything else) and saying that he launched the counter-offensive against the invading Saxons that ended in the Battle of Badon. Gildas does not mention Aurelius again by name after the early days of the fighting, and it is unclear if Gildas meant for his readers to assume that Aurelius was at the battle, or if it was led by one of his heirs.