A Byzantine-Mongol Alliance occurred during the end of the 13th and the beginning of the 14th century between the Byzantine Empire and the Mongol Empire. Byzantium actually tried to maintain friendly relations with both the Golden Horde and the Ilkhanate realms, who were often at war with each other. The alliance involved numerous exchanges of presents, military collaboration and marital links, but dissolved in the middle of the 14th century.

Soon after the Battle of Köse Dağ in 1243, the Empire of Trebizond surrendered to the Mongol Empire while the court of Nicaea put its fortresses in order. In the early 1250s, the Latin emperor of Constantinople Baldwin II sent an embassy to Mongolia in the person of the knight Baudoin de Hainaut, who, following his return, met in Constantinople with the departing William of Rubruck. William of Rubruck also noted that he met an envoy of John III Doukas Vatatzes, Emperor of Nicaea, at the court of Möngke Khan in circa 1253.

Emperor Michael VIII Palaiologos, after re-establishing Byzantine Imperial rule, established an alliance with the Mongols, who themselves were highly favourable to Christianity, as a minority of them were Nestorian Christians. He signed a treaty in 1266 with the Mongol Khan of the Kipchak (the Golden Horde), and he married two of his daughters (conceived through a mistress, a Diplovatatzina) to Mongol kings: Euphrosyne Palaiologina, who married Nogai Khan of the Golden Horde, and Maria Palaiologina, who married Abaqa Khan of Ilkhanid Persia.