Nanowire-based devices can potentially be of use in a variety of electronic applications, from ultrascaled digital circuits to 5G communication networks. However, the devices are typically restricted to low-power applications due to the relatively low electrical conductivity and limited voltage capability of the nanowires. Here, we show that wide-band-gap AlGaN/GaN nanowires containing multiple two-dimensional electron gas channels can be used to create high-electron-mobility tri-gate transistors for power-conversion applications. The multiple channels lead to improved conductivity in the nanowires, and a three-dimensional field-plate design is used to manage the high electric field. Power devices made with 15-nm-wide nanowires are shown to exhibit low specific on resistances of 0.46 m omega cm(-2), enhancement-mode operation, improved dynamic behaviour and breakdown voltages as high as 1,300 V. AlGaN/GaN nanowires containing multiple two-dimensional electron gas channels can be used to create tri-gate power transistors that overcome trade-offs between carried density and mobility.