The Ahl al-Ra'y, sometimes referred to in English as rationalists, refers to an Islamic creedal group advocating for the use of reason for theological decisions and scriptural interpretation. They were one of two main groups debating the source of Islamic creed in the second century of Islam, the other being Ahl al-Hadith (the people of hadith).

Its proponents, which included many early jurists of the Hanafi school, used the term ra'y to refer to "sound" or "considered" reasoning, such as qiyas (analogical deduction). Their opponents from the Ahl al-Hadith creedal group held that the Quran and authentic hadith were the only admissible sources of Islamic law, and objected to any use of ra'y in jurisprudence, whether in the form of qiyas, istislah (consideration of public interest), or hiyal (legal subterfuges). According to Daniel W. Brown, Ahl al-Ra'y thought a hadith should "sometimes be subject to other overriding principles" such as the "continuous practice" of the community and "general principles of equity" which was claimed to "better represent the spirit of the Prophet" Muhammad.

Over time, Hanafi jurists gradually came to accept the primacy of the Quran and hadith advocated by the Ahl al-Hadith creedal group, restricting the use of other forms of legal reasoning to interpretation of these scriptures. In turn, Hanbali jurists, who had led the Ahl al-Hadith creedal group, gradually came to accept the use of qiyas.

Ra'y is an Arabic word that literally means reason, opinion, idea, and other similar words. According to Lisan al-Arab, ra'y was used to refer to an excellent opinion in Pre-Islamic Arabia. Later definitions used it to refer to an opinion derived from deep contemplation and sound judgement. Those who prioritized the usage of ra'y in Islamic law became known as Ahl al-Ra'y or Așḥāb al-Ra'y, sometimes referred to in English as rationalists. Other names include Ahl al-Qiyās, Ahl an-Naẓar, and Ahl al-Kalām. Sometimes Ahl al-Kalām is considered its own separate category, containing the Mu'tazila and rejectors of hadith.

Ahl al-Ra'y did not necessarily reject all hadith. Rather, many accepted the usage of hadith but nonetheless, gave preference and greater importance to ra'y. To Ahl al-Ra'y, it was just one source of Islamic law among many, not necessarily given any more importance than other sources like qiyas or istihsan. They believed that hadith must sometimes be subjected to general religious principles like the "continuous practice" of the community and "general principles of equity" which was claimed to "better represent the spirit of the Prophet" Muhammad.

Ra'y originally was viewed in a very positive sense to refer to sound and considered opinion based on reasoning. The emergence of hadith did not initially affect these established forms of legal reasoning, and ra'y continued to dominate the Islamic world until the mid-8th century CE. About two-thirds of Zuhrī's transmitted doctrine contained ra'y and only one third contained reports from earlier authorities. For Qatada ibn Di'ama, 62% of his transmitted doctrine contained ra'y and of what was remaining, 84% was the ra'y of previous authorities.

However, starting around the end of the 7th century, there had been a growing movement to codify the sunnah into written hadith, rather than being largely verbally told by scholars and storytellers. It became a common practice amongst some scholars to "travel in search of knowledge" (talab al-'ilm). In this case, the "search for knowledge" was searching throughout the caliphate for textual sources that could be used for the compilation of hadith. While ra'y was initially the dominant source of Islamic law, this would soon change with the rise of traditionalism (Ahl al-Hadith), as the term got an increasingly negative association with arbitrary or fallible human thought. The beginning of this decline seems to start with Umayyad Caliph Umar II, ordering that any judge he appointed resort to 'ilm instead of ra'y when in doubt.

Traditionalists would at times claim ra'y was imposing human subjectivity onto God, while rationalists would at times claim much of the hadith materials, especially akhbār al-āḥād (single-transmitter reports), to have too uncertain accuracy. Traditionalists would often be accused of being primitive while rationalists were accused of being impious. In contrast to previous centuries where few jurists were seen as traditionalists (and most had acquired this description after the fact), the 9th century had many jurists identify clearly as traditionalists. The traditionalists had experienced an "unprecedented upsurge" by the last quarter of the 8th century, and by the mid-9th century, "ḥadīth had won the war against ra'y". Conversion from the rationalist to traditionalist camp was "frequent" while the reverse was "rare to nonexistent". By the 9th century, the term ra'y itself even lost any ground in legal discourse, with the terms qiyas and ijtihad being favored, having not gained a negative connotation.