We demonstrate the self-assembled growth of vertically aligned GaN nanowire ensembles on a flexible Ti foil by plasma-assisted molecular beam epitaxy. The analysis of single nanowires by transmission electron microscopy reveals that they are single crystalline. Low-temperature photoluminescence spectroscopy demonstrates that, in comparison to standard GaN nanowires grown on Si, the nanowires prepared on the Ti foil exhibit a equivalent crystalline perfection, a higher density of basal-plane stacking faults, but a reduced density of inversion domain boundaries. The room-temperature photoluminescence spectrum of the nanowire ensemble is not influenced or degraded by the bending of the substrate. The present results pave the way for the fabrication of flexible optoelectronic devices based on GaN nanowires on metal foils.The integration of electronic and optoelectronic devices on flexible substrates is motivated by the vision of novel and/or economically relevant applications. 1,2 In this context, inorganic semiconductor nanowires (NWs) have recently emerged as promising candidates for flexible electronics and optoelectronics. 2,3 Their large aspect ratio facilitates the expulsion of threading dislocation at the NW sidewalls, leading to their complete absence in the upper sections of well-developed NWs. This effect alleviates the requirement of lattice matching with the substrate and thus lifts the stringent constraints concerning the choice of the substrate material. 4,5 Indeed, the direct growth of semiconductor NW ensembles on flexible substrates has already been demonstrated for different material systems: ZnO NWs on polymer-based indium-tin-oxide coated substrates 6 and on paper, 7 Ge NWs on plastic films 8 as well as on Au coated Al foils 9 and Si NWs on stainless steel. 10 However, the direct growth of inorganic semiconductors on flexible substrates is frequently hampered by either the low melting point of the organic materials used as substrate 11 or the occurrence of interfacial reactions when semiconductors are directly grown on metallic surfaces. 12,13 To overcome these issues and extend the possible range of material combinations, the wet etching of the substrate followed by NW embedding in a plastic film, 14 and different lift-off techniques such as dry transfer 15,16 and transfer printinting 17,18 have been developed. Using these methods, arrays of vertically oriented CdS 14 and Si 11,19 NWs have been fabricated on flexible substrates. Regarding group-III nitrides, the material of choice for solid-state lighting and high-power electronics, the integration of GaN NWs on flexible substrates has been achieved either by transferring NW arrays grown on Si to polymer films 20,21 or by a lift-off process of NW ensembles grown on graphene coated Cu substrates. 22 Metal foils are a particularly interesting type of substrate because they are not only flexible but also exhibit excellent electrical and thermal conductivities as well as a high optical reflectance. However, as pointed out above, the direct growth of...