Perfluorosulfonate ionomer (PFSI) membranes are widely used in electrochemical applications such as fuel cells, chlor-alkali cells, and actuators/sensors. In this work, the surface crystallization of a solution-cast PFSI membrane is quantitatively characterized for the first time by synchrotron grazing incidence X-ray diffraction (GIXRD) and small angle X-ray scattering (GISAXS). A crystallite-rich skin is observed on the PFSI membrane surface, which is 4 nm thick, possesses a crystallinity which is 40% higher than the bulk, with the crystallites aligned parallel to the membrane surface. An intermediate layer exists between the skin and the bulk. The discovery of a crystallite-rich skin may help to explain the surface properties of PFSI membranes, and facilitate an understanding of the transport properties across the membrane interface.