This contribution explores the attitudes of el-Hajj Beshir Agha (term 1717-46), the most powerful Chief Harem Eunuch in Ottoman history, toward Sufism and Shi‘ism, with a view toward placing his predilections in historical context while gauging the representativeness of his attitudes and their effects on Ottoman society. The chief sources for this study are the deeds (singular, waqfiyya) of Beshir Agha’s pious endowments and select Ottoman chronicles composed during his lifetime. Beshir Agha’s affinity for “mainstream” Sufism centered on membership in a Sufi order (tarīqa) is evident from his endowments in Istanbul. His religious complex (külliye) outside Istanbul’s Topkapı Palace includes a lodge (tekke) for Sufis of the Naqshbandi order, which was well-established in the Ottoman Empire, although by the early eighteenth century, it was arguably more popular in the Ottoman Arab provinces than in the empire’s central lands. Southwest of his complex, in Istanbul’s Koca Mustafa Pasha neighborhood, Beshir Agha funded renovations to the tomb of Sünbül Efendi (d. 1529), an influential shaykh of the Khalwati (Halveti) Sufi order, which was one of the most widespread and popular in the Ottoman Empire, with branches in the capital and virtually every province. The impression of Sufi affinities is reinforced by the book catalogue from the library at Beshir Agha’s religious complex, which lists numerous mystical works. In contrast, Beshir Agha’s attitude toward Shi‘ism, particularly Twelver Shi‘ism as practiced in Safavid Iran, was uncompromisingly hostile. The endowment for a dār al-hadīth, or school for the study of sayings of the Prophet Muhammad, that he founded in Medina insists that students must be bachelors from Anatolia and stipulates that Shi‘ites – called revāfiz, or “heretics,” in the document – must not be admitted, along with Persians in general, North Africans, and Indians. This antagonism toward Iranian-style Twelver Shi‘ism appears again in accounts of Beshir Agha’s intervention in Ottoman negotiations with Nadir Shah, the Safavid revivalist who conquered parts of Ottoman Iraq in the 1740s. According to a contemporary chronicler, Beshir angrily rejected Nadir’s proposal that the Ottomans accept Twelver Shi‘ism as a fifth Sunni legal rite (madhhab). Overall, Beshir Agha’s attitudes reflect the rapprochement between tarīqa Sufism and Sunni orthodoxy that had emerged in Ottoman society by the mid-eighteenth century, as well as the ongoing polarization between Ottoman Sunnism and Iranian Shi‘ism – a polarization that was, however, more geo-political than doctrinal in nature.