Patterning of colloidal particles with chemically or topographically distinct surface domains (patches) has attracted intense research interest(1-3). Surface-patterned particles act as colloidal analogues of atoms and molecules(4,5), serve as model systems in studies of phase transitions in liquid systems(6), behave as 'colloidal surfactants'(7) and function as templates for the synthesis of hybrid particles(8). The generation of micrometre- and submicrometre-sized patchy colloids is now efficient(9-11), but surface patterning of inorganic colloidal nanoparticles with dimensions of the order of tens of nanometres is uncommon. Such nanoparticles exhibit size-and shape-dependent optical, electronic and magnetic properties, and their assemblies show new collective properties(12). At present, nanoparticle patterning is limited to the generation of two-patch nanoparticles(13-15), and nanoparticles with surface ripples(16) or a 'raspberry' surface morphology(17). Here we demonstrate nanoparticle surface patterning, which utilizes thermodynamically driven segregation of polymer ligands from a uniform polymer brush into surface-pinned micelles following a change in solvent quality. Patch formation is reversible but can be permanently preserved using a photocrosslinking step. The methodology offers the ability to control the dimensions of patches, their spatial distribution and the number of patches per nanoparticle, in agreement with a theoretical model. The versatility of the strategy is demonstrated by patterning nanoparticles with different dimensions, shapes and compositions, tethered with various types of polymers and subjected to different external stimuli. These patchy nanocolloids have potential applications in fundamental research, the self-assembly of nanomaterials, diagnostics, sensing and colloidal stabilization.