Basil I (known as the Macedonian, 811 – August 29, 886), Byzantine emperor, was born to a family of Greek (not Slavonic) descent, settled in Macedonia.

He spent a part of his childhood in captivity in Bulgaria, where his family had lived as captives of the Bulgarian prince Krum since 813. He succeeded in escaping and was ultimately lucky enough to enter the service of Theophilitzes, a relative of the Caesar Bardas (uncle of Michael III 'the Drunkard'), as groom. While serving Theophilitzes he visited the city of Patrae, where he gained the favour of Danielis, a wealthy woman who took him into her household and endowed him with a fortune. He also earned the notice of Michael III by winning a victory in a wrestling match, and soon became the emperor's companion and bodyguard (parakoimomenos).

To gain favour with Michael, he divorced his wife and married Eudocia Ingerina, one of Michael's mistresses. It was commonly believed that Leo VI, Basil's successor and reputed son, was really the son of Michael. Basil then murdered Bardas, who virtually ruled the empire in Michael's place; this was done with the emperor's consent in April, 866, and a few weeks later Basil was named Caesar. Up to this point, it is unlikely that anyone imagined Basil would be capable of administering the empire alone, as he shared in the debauches of his friend Michael. In September, 867, he had Michael assassinated, and reigned alone.

He inaugurated a new age in the history of the empire, associated with the dynasty which he founded. This is called the Macedonian Dynasty. It was a period of territorial expansion, during which the empire was the strongest power in Europe.

Because of the great legislative work which Basil undertook, and which may be described as a revival of the laws of Justinian I, he is often called the "second Justinian." Basil's laws were collected in the Basilica, consisting of sixty books, and smaller legal manuals known as the Prochiron and the Eisagoge. Leo VI was responsible for completing these legal works. Basil's financial administration was prudent.

His ecclesiastical policy was marked by good relations with Rome. One of his first acts was to exile the patriarch Photius and restore his rival Ignatius , whose claims were supported by Pope Adrian II. However, Basil had no intention of yielding to Rome beyond a certain point. The decision of the Bulgarian tsar Michael to submit the new Bulgarian Church to the jurisdiction of Constantinople was a great blow to Rome, who had hoped to secure it for herself. In 877 Photius became patriarch again, and there was a virtual, though not a formal, breach with Rome. This was a watershed event in conflicts that led to the Great Schism that ultimately produced Roman Catholicism and the Orthodox Church as separate entities.

His reign was marked by a troublesome war with the Paulician heretics, an inheritance from his predecessor; the death of their chief Chrysochir led to the definite subjection of their state, centred on Tephrice on the Euphrates and aided by the Saracens. There was the usual frontier warfare with the Saracens in Asia Minor. Cyprus was recovered, but retained for only seven years. Syracuse was lost, but Bari and much of Calabria was recovered. The successes in Italy opened a new period of Byzantine domination there. Above all, the Byzantines were once again in control of the Mediterranean Sea, and especially the Adriatic.

Basil reigned nineteen years. His death on August 29, 886 was due to a fever contracted after a serious hunting accident, when his belt was caught in the antlers of a deer and he was dragged from his horse. He was saved by an attendant who cut him loose with a knife, but he suspected the attendant of trying to assassinate him and had the man executed shortly before he himself died.