The Bayt al-Māl (literally meaning the House for Money, is used as an equivalent for the Treasury House where public fund is kept) is an Islamic concept built upon three main factors: wealth, trust and socio-economic justice. It is clear that attainment of socio-economic justice is an immediate objective of the Bayt alMāl. The Bayt al-Māl bears the responsibility of the undertaking of the society’s fiscal system and welfare provisions. Accordingly, the Bayt al-Māl acts as a treasury complemented with the task of planning and distributing society’s wealth in the whole socio-economic and political set-up of the nation. Hence, it is the financial pulse to the development process of the nation. In this paper I intend to briefly consider the philosophy and sources of wealth in the Bayt al-Māl and then examine zakāt as its main part, elaborating on coverage, rates and the distribution of zakāt. Finally, I will try to show the role of zakāt in the socio-economic justice of Muslim countries.