This paper highlights how, in the Paris metropolitan region's case, two polycentric spatial planning policies at national and regional levels incidentally prevented the development of a polycentric Mega-City-Region. The national policy aiming to reduce economic imbalances in the French territory has failed to promote a coherent Bassin Parisien system. At the same time, polycentric planning within the Paris-Ile-de-France region restricted the economic and urban growth to the adjacent new towns. This resulted in the reinforcement of the Paris agglomeration, thus limiting further integration of surrounding FURs in a polycentric Mega-City-Region that never existed.