Custom is a source of Islamic law in general and Siyar (Islamic international law) in particular. Islamic jurists have set out the elements and conditions of customs for general jurisprudential purpose. However, no one has, to the authors’knowledge, formulated them from Siyar perspective. This paper is an attempt to fill this gap by tracing two important elements of an international custom, namely frequent and dominant general practice of States, and acceptance of that practice as law. These two will constitute a valid custom provided they fulfill certain conditions, most importantly that the custom must not conflict with Shari’ah or the spirit of Shari’ ah.