This article researches political activities of the son of the Ottoman Sultan Bayezid I (1389–1402) Mustafa Celebi, supported by the Byzantine Emperors Manuel II and John VIII Palaiologoi in the internecine war with Sultan Murad II (1421–1444, 1446–1451). The materials of Byzantine, Ottoman, and Venetian sources provide the background to analyze the degree of participation of the Byzantine basilei in the intra-dynastic affairs of the Ottomans, the reasons leading to the Byzantine support for Mustafa, and the consequences of the Ottoman internecine war for the Byzantine state. The study undertaken has discovered that the priority direction of Byzantine policy in the first quarter of the fifteenth century was to weaken the Ottoman house. To this end, Byzantine basilei actively intervened in the dynastic feuds of the Ottomans, supporting one of the candidates for the Ottoman throne, and used the Ottoman princes who were at the Byzantine court to influence the internal affairs of the Ottoman state. Byzantine emperors’ fragile alliance with one of the contenders for the Ottoman throne Mustafa Celebi, who was defeated in the intra-dynastic struggle with Sultan Murad II, not only did not bring significant dividends to the empire, but also aggravated the conflict between the imperial house and the sultan, which led to a new stage of Ottoman expansion into the Byzantine lands.