In this work the position-controlled growth of GaN nanowires (NWs) on diamond by means of molecular beam epitaxy is investigated. In terms of growth, diamond can be seen as a model substrate, providing information of systematic relevance also for other substrates. Thin Ti masks are structured by electron beam lithography which allows the fabrication of perfectly homogeneous GaN NW arrays with different diameters and distances. While the wurtzite NWs are found to be Ga-polar, N-polar nucleation leads to the formation of tripod structures with a zinc-blende core which can be efficiently suppressed above a substrate temperature of 870 °C. A variation of the III/V flux ratio reveals that both axial and radial growth rates are N-limited despite the globally N-rich growth conditions, which is explained by the different diffusion behavior of Ga and N atoms. Furthermore, it is shown that the hole arrangement has no effect on the selectivity but can be used to force a transition from nanowire to nanotube growth by employing a highly competitive growth regime. ; Financial support from the Deutsche Forschungsgemeinschaft (DFG) via the Forschergruppe 1493, TUM.solar in the frame of the Bavarian Collaborative Research Project “Solar technologies go Hybrid” (SolTec), and the excellence program Nanosystems Initiative Munich is gratefully acknowledged. The authors thank the group of C.E. Nebel at Fraunhofer IAF in Freiburg for supplying the SCD diamond substrates. J.A. acknowledges the funding from the Spanish MINECO MAT2014-51480-ERC (e-ATOM) and Generalitat de Catalunya 2014SGR1638. MdlM thanks the CSIC Jae-Predoc program. ; Peer Reviewed