The world faces today multiple challenges related to energy consumption and limited availability of resources. Buildings play an important role in this regard as they consume a great deal of these resources including energy. Efforts are being combined to improve energy efficiency of buildings of all types, including mosques. Retrofitting of mosque buildings is an important intervention that improves their sustainability and make them an example to be followed due to their spiritual and social importance. Therefore, this study aims to discuss the different strategies for retrofitting existing mosque buildings to make them more sustainable and less energy-consuming, taking the climatic and urban environment of Saudi Arabia as a case study. Numerical simulation of several strategies will be conducted in this regard including installation of external thermal insulation for roofs and walls, replacing windows, and taking advantage of natural lighting. It is hoped that this study will result in a set of recommendations for the design of contemporary mosques making them more sustainable.Keywords: Architecture; historical mosques; Palestine; post-war, reconstruction.