Starting from the Islamic Revolution, Iran's regime had increasingly been presented as the external source of Islamic fundamentalist movements in Turkey, leading to the establishment of a security-driven atmosphere in bilateral relations. However, Turkish-Iranian relations witnessed a positive change in the 2000s. The impact of the security discourse of Turkish elites regarding Iran's Islamic regime declined and a rapprochement process began. This research suggests that such shift in Turkey's foreign policy agenda became possible with the desecuritization of Islamic fundamentalism in Turkey's domestic politics. The article proposes a securitization framework to analyse the link between domestic politics and foreign policy. Accordingly, it asks 'How did the desecuritization of Islamic fundamentalism in Turkey's domestic politics influence its foreign policy towards Iran?' It analyses Turkey's Iran policy comparatively in the 1990s and 2000s by utilizing speeches of Turkey's political elites, public opinion in Turkey, changing the domestic political structure and alterations in foreign policy practices of Turkey towards Iran.