The present work reports high-quality nonpolar GaN/Al0.6Ga0.4N multiple quantum wells (MQWs) grown in core−shell geometry by metal−organic vapor-phase epitaxy on them-plane sidewalls ofc̅-oriented hexagonal GaN wires. Optical and structural studies reveal ultraviolet (UV) emission originating from the core−shell GaN/AlGaN MQWs. Tuning them-plane GaNQW thickness from 4.3 to 0.7 nm leads to a shift of the emission from 347 to 292 nm, consistent with Schrödinger−Poissoncalculations. The evolution of the luminescence with temperature displays signs of strong localization, especially for samples with thinner GaN QWs and no evidence of quantum-confined Stark effect, as expected for nonpolar-plane surfaces. The internal quantum efficiency derived from the photoluminescence (PL) intensity ratio at low and room temperatures is maximum (∼7.3%measured at low power excitation) for 2.6 nm thick quantum wells, emitting at 325 nm, and shows a large drop for thicker QWs. An extensive study of the PL quenching with temperature is presented. Two nonradiative recombination paths are activated at different temperatures. The low-temperature path is found to be intrinsic to the heterostructure, whereas the process that dominates at high temperature depends on the QW thickness and is strongly enhanced for QWs larger than 2.6 nm, causing a rapid decrease in the internal quantum efficiency.