This paper examines the role of mercury in “Graeco-Islamic” medicine, which is referred to as Ṭibb-e yūnānī or unani medicine in South Asia. Having its origin in Ancient Greece, unani medicine spread to the Arabic countries and from the fifteenth century onwards to India. With its main roots in the Greek and Latin sources, the most influential works of ‘ilm al-adviya (pharmacology) were translated into Arabic, Persian and Urdu. Mercury (Arabic: zībaq; Persian: sīmāb; Urdu: sīmāb and pāra) played an important role in all Indian traditions of medicine, and had a prominent place in unani medicine. This paper highlights the historical use of mercury in Indian, Persian and Urdu medical literature, the discourses on its efficacy and some of the important mercurial preparations presented in a selection of unani works. Further, the use of mercury as a single and compound drug and its role in the treatment of different diseases will be analysed.