This is a list of 57 member states of the Organisation of Islamic Cooperation sorted by their estimated population.

10 countries (Benin, Cameroon, Gabon, Guinea-Bissau, Guyana, Ivory Coast, Mozambique, Suriname, Togo and Uganda) are not Muslim-majority, but they are members of Organisation of Islamic Cooperation.

Also the list below only includes OIC members, not OIC observers, and does not include many other countries having a significant number of Muslims in their resident population from immigrants and their descendants (notably in North America, Europe, and even in Asia, such as China, India and Russia which are OIC observers).

Syria is still listed here (as a founding member state), even if its membership is suspended since 2022 (other member states have also been suspended in the past but fully reintegrated a few years later in the OIC).^{[lower-alpha 1]}