This paper explores how the Prophetic ḥadīth “The caliphate (khilāfa) after me will be thirty years, followed by kingship (mulk)” (also known as the “thirty-year ḥadīth”) was discussed and understood in medieval Sunni theological discourses (kalām) on the caliphate/imamate, Qurʾānic exegeses (tafsīr), and ḥadīth commentaries. The thirty-year ḥadīth and its variants—all related on the authority of Safīna (a mawla of the Prophet)—form the crux of Muslim debates on the distinction between legitimate caliphate and worldly kingship. Several scholars today have taken the saying to mean that only the Rāshidūn caliphs had been caliphs in the full sense of the word; the Umayyads and Abbasids after them were mere kings or pseudo-caliphs at best. The founder of the Republic of Turkey, Mustafa Kemal, even quotes it in support of his argument that the caliphate was not an essential institution in Islam and should therefore be abolished. Aside from its Baṣran and Wāsiṭī isnāds, the thirty-year ḥadīth’s origins remain an open question, though Muhammad Qasim Zaman has recently argued that it began to be circulated in the second/eighth century under the Abbasids to legitimize the position of ʿAlī as one of the four Rāshidūn caliphs. The ḥadīth was eventually included in the ḥadīth compilations of Aḥmad b. Ḥanbal (d. 241/855), Abū Dāwūd (d. 275/889), and al-Tirmidhī (d. 279/892), and would be brought up for discussion in subsequent centuries by Sunni theologians writing about the caliphate. This paper is less concerned with the ḥadīth’s provenance, but more with how it was understood by medieval Sunni thinkers in different times and places. I argue that far from evoking the thirty-year ḥadīth as a proof text to delegitimize the Abbasids whom they were living under, Sunni theologians writing about this ḥadīth often engaged in nuanced discussions of the legitimacy of the post-Rāshidūn caliphs. Though cited to legitimize the Rāshidūn caliphs and to discuss the legitimacy of Muʿāwiya (d. 60/680) as “caliph,” discussions of the ḥadīth went beyond drawing a clear demarcating line between the Rāshidūn and post-Rāshidūn caliphs, as observed in al-Āmidī’s (d. 631/1233) discussion of the caliphate in his Abkār al-afkār and al-Taftāzānī’s (d. 793/1390) commentary on the creed of al-Nasafī (d. 537/1142). I also examine the change over time in the ways by which the thirty-year ḥadīth was discussed, and link these ideological shifts to wider political developments surrounding each thinker.