With rapid development towards shrinking the size of traditional photonic systems such as cameras, spectrometers, displays and illumination systems, there is an urgent need for high performance and ultra-compact functional optical elements. The large footprint of traditional bulky optical elements, their monofunctional response and the inability for direct integration into nanophotonic devices have severely limited progress in this area. Metasurfaces, consisting of an array of subwavelength nanoscatterers with spatially varying geometries, have shown remarkable performance as ultrathin multifunctional optical elements. Here, based on an all-dielectric metasurface, we propose and experimentally demonstrate a spatial domain optical depolarizer capable of efficiently depolarizing linearly polarized light in the visible spectral band from 450 nm to 670 nm, with a degree of polarization of less than 10 %. Remarkably, it is capable of depolarizing light beam with a diameter down to several micrometers, about two orders of magnitude smaller than commercial liquid crystal depolarizers. Furthermore, the long response time, bulky footprint, tight optical alignment tolerance and large pixel size severely limit the performance and system integration of commercial depolarizers. We envision the metasurface depolarizer to find applications in next generation ultra-compact grating spectrometers and illumination systems.