In the history of Islamic thought, Sayyid Bahāʾ al-Dīn Ḥaydar al-Āmulī (d. after 787/1385) is primarily remembered as one of the earliest figures to fully integrate Ṣūfism, especially the Ṣūfī intellectual tradition associated with Muḥyī al-Dīn Ibn ʿArabī (d. 638/1240) and his followers, within a Twelver Shīʿī framework. Āmulī’s systematic synthesis of the Ṣūfī and Shīʿī traditions led him to formulate and defend a vision of esoteric orthodoxy according to which the secret and superior aspect of tawḥīd—the most fundamental doctrine of Islamic monotheism—is known only to an elite community at the very apex of the spiritual hierarchy. This esoteric tawḥīd, which Āmulī refers to as ‘ontological tawḥīd,’ is in fact a reformulation of the highly controversial doctrine of ‘the oneness of existence’ (waḥdat al-wujūd); analogously to the exoteric tawḥīd expressed in the shahāda, which denies any divinity other than Allāh, esoteric-ontological tawḥīd holds that existence in an absolute sense belongs only to God, with everything other than God being the particularized disclosures (tajallīyāt) of God’s absolute existence. Āmulī’s theory of tawḥīd is the key to understanding his entire worldview. In this paper, I explore how his theory of tawḥīd informs his universal hermeneutics, which is methodologically directed towards interpreting the correspondences (taṭbīqāt) between all domains of reality, conceived of as three great ‘books’: the Qurʾān, the created world, and humanity. This will be accomplished through a reading and analysis of passages from his voluminous Tafsīr al-Muḥīṭ al-aʿẓam. Though now only partially extant, this work represents a comprehensive summa of Āmulī’s thought, and in it he most fully articulates his universal hermeneutic methodology. In addition to outlining Āmulī’s universal hermeneutics, I endeavor to contextualize his theory in relation to broader patterns in Islamic intellectual history. While his hermeneutic theory is, in a sense, quite original, it also clearly participates in a widespread épistémè of the time that viewed the cosmos as a divinely revealed scripture; such ideas were current in a variety of discursive spheres, including Ṣūfism (especially among Ṣūfīs sympathetic to Ibn ʿArabī), Shīʿism (especially among Ismāʿīlīs and those who were pejoratively denounced as ghulāt or ‘extremists’), and occultism (especially practitioners of the science of letters). I conclude my paper by examining how Āmulī put his universal hermeneutics into practice. To do so I offer a reading of his exegesis of Sūrat al-fātiḥa contained in the only extant portion of the Tafsīr to directly explicate the Qurʾān.