The identity of Islamic cities is distinctly manifested through their congregational mosques, primarily characterized by their continuous and stable function, epitomized by the five daily congregational prayers. The performance of a mosque is intricately linked to a set of religious, organizational, structural, and economic determinants. and it is closely tied to the functional and architectural attributes of its elements and spaces. The historic congregational mosques in the Islamic world have predominantly adhered to the architectural model of the Prophet's Mosque, characterized by its rectangular form. In modern mosques, various factors such as technological advancements, and cultural and scientific influences have led to architectural trends and stylistic innovations. These innovations have aimed to enhance the efficiency and performance of congregational mosques, one of which is the octagonal form. The research problem centred on the utilization of the octagonal shape and its adaptability in the context of the congregational mosques in the city of Mosul and its suitability for optimal mosque functioning. The research sought to elucidate the appropriateness of employing the octagonal form in congregational mosques by analysing samples with octagonal layouts in Mosul to investigate the extent to which this form influenced the characteristics of the mosque. The research concluded that all octagonal layouts had undergone modifications and expansions to rectify the unsuitable layout, indicating that the octagonal form is foreign to the traditional mosque typology. Furthermore, it is subject to additional alterations to accommodate the functional and architectural requirements of the congregational mosque.