Basiliscus (Ancient Greek: Βασιλίσκος, romanized: Basilískos; died 476/477) was Eastern Roman emperor from 9 January 475 to August 476. He became magister militum per Thracias in 464, under his brother-in-law, Emperor Leo I (r. 457–474). Basiliscus commanded the army for an invasion of the Vandal Kingdom in 468, which was defeated at the Battle of Cape Bon. There were accusations at the time that Basiliscus was bribed by Aspar, the magister militum; many historians dismiss this, instead concluding that Basiliscus was either incompetent or foolish for accepting Vandal King Gaiseric's offer of a truce, which the latter used to construct fireships. Basiliscus's defeat cost the Eastern Empire 130,000 pounds (59,000 kg) of gold, causing the empire to hover above bankruptcy for 30 years. When Basiliscus returned to Constantinople, he sought refuge in the Hagia Sophia. His sister, Empress Verina, secured him a pardon and he left the church to retire in Neapolis.

When Emperor Leo died in 474, his grandson Leo II (r. 474) took power, but soon died; his father, Zeno (r. 474–475, 476–491) ascended the throne in the same year, in a politically precarious position. Verina conspired to install the magister officiorum Patricius, her lover, as emperor. This plot was supported by Basiliscus, who succeeded in recruiting Isaurian brothers Illus and Trocundes, as well as Verina's nephew Armatus. Zeno fled on 9 January 475, either after learning of the plot or after Verina warned him that his life was in danger. Although Patricius was Verina's intended successor, Basiliscus convinced the Eastern Roman Senate to acclaim him instead.

Basiliscus quickly lost the support of his subjects and of his allies in court. Verina was alienated by the execution of Patricius, while the people objected to a combination of heavy taxes, heretical policies, and a natural disaster viewed as divine wrath for said heretical views. In an attempt to increase support, Basiliscus embraced the miaphysites, restoring Timothy Ailuros as the Patriarch of Alexandria and Peter the Fuller as Patriarch of Antioch. He heeded their advice and issued an encyclical on 9 April 475 which promoted the first three ecumenical councils of the church: Nicaea, Constantinople, and Ephesus, and condemned the Council of Chalcedon and the Tome of Leo. The Patriarch of Constantinople, Acacius, strongly opposed him, and together with Daniel the Stylite, turned the population of Constantinople against Basiliscus.

Zeno, besieged by Illus and Trocundes in his homeland of Isauria, convinced the two generals to defect, and soon the three of them marched their troops toward the capital. Basiliscus ordered Armatus to intercept them, but Armatus also switched sides after Zeno promised to give him the position of magister militum praesentalis for life, and make his son Basiliscus caesar. With Armatus' defense force deliberately kept out of his way, Zeno entered Constantinople unopposed in August 476. Basiliscus and his family hid in a church until Zeno promised not to execute them; exiled to Limnae in Cappadocia, they were either beheaded or imprisoned in a dried-up cistern and left to starve to death.

Basiliscus was born at an unknown date, likely in the Balkans. He was the brother of future Empress Verina, the wife of East Roman Emperor Leo I (r.  457–474). Historian Stefan Krautschick argues that since Basiliscus' nephew, Armatus, was the brother of a barbarian named Odoacer, Basiliscus was therefore also the uncle of Odoacer and thus a barbarian; this argument has been opposed by historians Wolfram Brandes, and Hugh Elton. Elton remarks that Krautschick's argument relies upon a single fragmentary Greek source, making his argument acceptable, but ignores the total lack of contemporary sources mentioning his ethnicity or relationship to Odoacer. Basiliscus married Zenonis, and with her produced a son, Marcus. Zenonis' origins are unknown, but she may have been a miaphysite, as some authors credit her for pushing Basiliscus's religious policies. Basiliscus was also related by marriage to Emperor Julius Nepos (r. 474–475/480), as the uncle of Julius Nepos's wife. Zenonis is alleged to have taken Armatus, Basiliscus's nephew, as a lover. Byzantist J.B. Bury, summarizing sources from the Suda, Candidus, and Malchus states that:

Basiliscus permitted Armatus, inasmuch as he was a kinsman, to associate freely with the Empress Zenonis. Their intercourse became intimate, and as they were both persons of no ordinary beauty they became extravagantly enamoured of each other. They used to exchange glances of the eyes, they used constantly to turn their faces and smile at each other; and the passion which they were obliged to conceal was the cause of dule and teen. They confided their trouble to Daniel, an eunuch, and to Maria, a midwife, who hardly healed their malady by the remedy of bringing them together. Then Zenonis coaxed Basiliscus to grant her lover the highest office in the city.

Leo rose to the throne of the Eastern Roman Empire in 457 after the death of Emperor Marcian (r. 450–457). Aspar, the magister militum, selected him for this position, much as he had selected Marcian himself. Despite being half-Alanic and half-Gothic, Aspar held much influence in the empire since the mid fifth century, and wielded significant power over Marcian and Leo. Basiliscus served as a military officer under Leo, and was made consul of the East for 465, with Flavius Hermenericus as consul of the West. Basiliscus was granted the rank of magister militum per Thracias in c. 464, and held it till 467/468; during this period he won many victories in Thrace against the Huns and Goths. He led troops in a war against a mixed group of the two in 466/467, along with Anagast, Aspar, and Ostrys. He was made a patricius at some point, first mentioned as such in 468.

Around 466, Leo attempted to free himself of Aspar's control. He utilized the support of the Isaurians, marrying the Isaurian chieftain Zeno (r. 474–475, 476–491) to his daughter Ariadne, in exchange for support. This resulted in a significant change to eastern imperial policy, notably ending the policy of ignoring the requests from the West for military aid. After Anthemius (r. 467–472), Marcian's son-in-law, was installed by Leo as Western emperor on 12 April 467, an embassy was sent to the Vandal King Gaiseric, to inform him as well as warn him not to interfere in Italy or the Western Roman throne; Gaiseric accused them of violating a prior treaty, possibly enacted in 433, and prepared for war. Rumors may have spread in Constantinople that the Vandals were preparing an invasion of Alexandria. As a result of this, in 468 Basiliscus was given command of an expedition against the Vandals, and likely promoted to magister militum praesentalis at that time. It is said that the fleet consisted of 1,113 ships, with more than 100,000 men under the command of Basiliscus, including mercenaries from as far as Öland, Sweden. According to Bury, Leo was influenced by both Verina and Aspar in his selection of Basiliscus, whom he describes as both "incompetent and untrustworthy"; he further states that Aspar intentionally chose a poor commander, to prevent Leo from becoming stronger by way of weakening the Vandals. Historians Gerard Friell and Stephen Williams dismiss this, but accept that Verina pushed for his appointment and that Aspar did not object. Historian Peter Heather points out that, at this point, Basiliscus had just returned from considerable success on the Balkan frontier of the empire.