Alexius I (1048–August 15, 1118), Byzantine emperor (1081–1118), was the third son of John Comnenus, nephew of Isaac I Comnenus (emperor 1057–1059).

His father declined the throne on the abdication of Isaac, who was accordingly succeeded by four emperors of other families between 1059 and 1081. Under one of these emperors, Romanus IV Diogenes (1067–1071), he served with distinction against the Seljuk Turks. Under Michael VII Parapinaces (1071–1078) and Nicephorus III Botaniates (1078–1081) he was also employed, along with his elder brother Isaac, against rebels in Asia Minor, Thrace and in Epirus in 1071.

The success of the Comneni roused the jealousy of Botaniates and his ministers, and the Comneni were almost compelled to take up arms in self-defence. Botaniates was forced to abdicate and retire to a monastery, and Isaac declined the crown in favour of his younger brother Alexius, who then became emperor at the age of 33.

His long reign of nearly 37 years was full of struggle. At the very outset he had to meet the formidable attack of the Normans (Robert Guiscard and his son Bohemund), who took Dyrrhachium and Corfu, and laid siege to Larissa in Thessaly (see Battle of Dyrrhachium). The Norman danger ended for the time with Robert Guiscard's death in 1085, and the conquests were reversed.

He had next to repel the invasions of Pechenegs and Cumans in Thrace, with whom the Manichaean sect of the Bogomils made common cause; and thirdly, he had to cope with the fast-growing power of the Seljuk Turks in Asia Minor.

Above all he had to meet the difficulties caused by the arrival of the knights of the First Crusade, which had been, to a great degree, initiated as the result of the representations of his own ambassadors, whom he had sent to Pope Urban II at the Council of Piacenza in 1095. The help which he wanted from the West was simply mercenary forces and not the immense hosts which arrived, to his consternation and embarrassment. The first group, under Peter the Hermit, he dealt with by sending them on to Asia Minor, where they were massacred by the Turks in 1096.

The second and much more serious host of knights, led by Godfrey of Bouillon, he also led into Asia, promising to supply them with provisions in return for an oath of homage, and by their victories recovered for the Byzantine Empire a number of important cities and islands—Nicaea, Chios, Rhodes, Smyrna, Ephesus, Philadelphia, Sardis, and in fact most of Asia Minor (1097–1099). This is ascribed by his daughter Anna as a credit to his policy and diplomacy, but by the Latin historians of the crusade as a sign of his treachery and falseness. The crusaders believed their oaths were made invalid when Alexius did not help them during the siege of Antioch; Bohemund, who had set himself up as Prince of Antioch, briefly went to war with Alexius, but agreed to become Alexius' vassal under the Treaty of Devol in 1108.

During the last twenty years of his life he lost much of his popularity. The years were marked by persecution of the followers of the Paulician and Bogomil heresies—one of his last acts was to burn Basilius, a Bogomil leader, with whom he had engaged in a theological controversy; by renewed struggles with the Turks (1110–1117); and by anxieties as to the succession, which his wife Irene wished to alter in favour of her daughter Anna's husband, Nicephorus Bryennius, for whose benefit the special title panhypersebastos ("honored above all") was created. This intrigue disturbed even his dying hours.

He deserves the credit for having saved the Empire from a condition of anarchy and decay at a time when it was threatened on all sides by new dangers. No emperor devoted himself more laboriously, or with a greater sense of duty, to the task of ruling.