Some argue that the earliest case of Obsessive–Compulsive Disorder (OCD) was reported by Robert Burton in his compendium The Anatomy of Melancholy (1621) and that only in the 19th century did modern concepts of OCD evolve, differentiating it from other types of mental illness. In this paper, we aim to reveal an even earlier presentation of the malady we now call OCD based on the 9th century work, Sustenance of the Body and Soul, written by Abu Zayd al-Balkhi during the Islamic Golden Era. Discovery of this manuscript reveals that Abu Zayd al-Balkhi should be credited with differentiating OCD from other forms of mental illnesses nearly a millennium earlier than is currently claimed by anthologies documenting the history of mental illness. Particular attention is paid to al-Balkhi′s classifications, symptom descriptions, predisposing factors, and the treatment modalities for obsessional disorders. Analysis of this manuscript in light of the DSM-5 and modern scientific discoveries reveals transcultural diagnostic consistency of OCD across many centuries. Theoretical and clinical implications of these findings are also discussed.