A collection of factors underlay Khwajah Naṣīr al-Dīn Ṭūsī’s association with and dissociation from the Ismā‘īlī sect. This paper aims to disclose the role of his religious beliefs in his interactions with this religious sect. Ṭūsī’s education was completed in an atmosphere of Twelver Shi‘ite philosophy, but in his youth he was also disenchanted with the existing imitative beliefs and dogmatic emphasis on extrinsic features of Sharī‘ah. This led him to develop an interest in some Ismā‘īlī teachings such as their attention to the esoteric meaning of religious texts and join this sect in response to their invitation. Of course, the undesirable conditions in the political-religious geography of the east of the Islamic world had also limited Ṭūsī’s choices. Nevertheless, based on some historical reports of the time of his relationship with the Ismā‘īlīs and some of his works that had been written in conformity with Ismā‘īlī ideas, it can be said that he had some ideological disagreements with them after joining the sect. His reaction after his separation from the Ismā‘īlīs, whom he has introduced as atheists and non-Muslims in his Kalāmī books, demonstrate his ideological conflicts with this sect. In fact, Ṭūsī joined the Ismā‘īlīs in his youth because of his religious ideas and, later, separated from them for the same reason. He revealed his ideological opposition to them in his Kalāmī written works.