 It is very well known that the medieval Islamic civilization played a pivotal role in the historical development of technical aspects of mathematics. Standing on the shoulders of their pre-Islamic Greek, Indian, and Persian ancestors, Muslim mathematicians have made numerous innovations in various branches of mathematics and have written a great number of books and essays introducing mathematical notions and proving mathematical theorems (Al-Daffaʾ 1977; R. Rashed 1984b [1994]; 1996 [2012]; 2015; Berggren 2016). However, it is very difficult (if not impossible) to find a book from the medieval Islamic era that is exclusively devoted to a comprehensive and systematic study of philosophy of mathematics. Nevertheless, many of the aforementioned philosophical issues about mathematics have been addressed, by great Muslim medieval thinkers, in various works whose central subject was mathematics, physics, metaphysics, or even theology (kalām). Putting these scattered engagements together, it becomes clear that although philosophy of mathematics has never been treated as an independent discipline in the medieval Islamic world, Muslim thinkers came up with very interesting and profound ideas, insights, and arguments about at least some philosophical issues related to mathematics. This entry briefly reviews the most remarkable examples of such insights and arguments, some of which have been the subject of long-standing debates and discussions among Muslim thinkers. Accordingly, the technical mathematical works of Arab and Muslim scholars will be discussed only to the extent that they contain materials related to the philosophy of mathematics.