Muslim sources agree that the conversion of Tarmashirin Khan paved the way to the overall Islamization of the Khanate of Chaghadai, the Mongol state in Central Asia. Yet apart from that, there is very little agreement among the sources: even the dates of Tarmashirin's reign and the extent of his realm are not unequivocally established. Moreover, unlike other Chinggisids who won their fame as the Islamizers of their realms, Tarmashirin's conversion engendered no conversion stories in the sources. On the basis of Muslim, Chinese, and numismatic sources, this article seeks to shed some light on the reign of Tarmashirin. It suggests a chronological framework for his career and, in the light of this framewoork, utilizing Tarmashirin's biography in the works of his contemporary, the Mamluk historian al-Scombining dot belowafadī (d. 1363), the paper reexamines Tarmashirin's Islam. What do we know about his conversion; how did his islamization affect his foreign and domestic policies; and what part did it play in his deposition? Lastly, the paper locates Tarmashirin's conversion in the general environment of Chaghadaid and Mongol islamization. His inglorious rule and the heavy shadow Tamerlane was soon to cast over the Chaghadaids explain why Tarmashirin never achieved a posthumous fame equivalent to that of Ghazan or Özbeg in their respective realms.