The present research aims to investigate whether atheism is present in the thought of medieval Muslim thinkers. So far, these thinkers have been usually disregarded or poorly addressed in studies about the historical evolution of atheism. Previous studies about the evolution of atheism have pointed out that this concept is expressed both through the rejection of a specific, usually the most dominant religion, as well as through claims that do not aim to reject the divinity, but that considerably limit the presence of the supernatural in human affairs. In order to identify whether atheism is present in medieval Islam, this study focuses on the thought of Ibn al-Rāwandī, al-Rāzī and al-Ma'arrī, three major representatives of the Muslim medieval intellectual milieu. They never rejected God explicitly, but they clearly doubted the possibility that Allāh existed, attributing traditional monotheistic views about Him to an invention of the prophets. What is more, their atheism appears evident in the view they had of the world. They believed in an essentially secular world, where the individual should seek a personal and collective realisation; human existence is not finalised to the realisation of divine plans, rather to the individual contribution to the creation of a better existence in the present moment; reason and critical thinking should never be subordinated to religious considerations; and, finally, morality is independent from religious considerations. These aspects, highlighted in the studies about the evolution of atheism in Western thought, are central elements of the Muslim medieval thinkers analysed as well. The research shows that during the period of the formation of the Islamic dogma, views that excluded God from the ways human existence was regulated were well known and debated among Muslim thinkers, anticipating of many centuries the European Enlightenment.