Difficulties in obtaining high-performance p-type transistors and gate insulator charge-trapping effects present two major challenges for III–V complementary metal–oxide semiconductor (CMOS) electronics. We report a p-GaAs nanowire metal–semiconductor field-effect transistor (MESFET) that eliminates the need for a gate insulator by exploiting the Schottky barrier at the metal–GaAs interface. Our device beats the best-performing p-GaSb nanowire metal–oxide−semiconductor field effect transistor (MOSFET), giving a typical subthreshold swing of 62 mV/dec, within 4% of the thermal limit, on–off ratio ∼105, on-resistance ∼700 kΩ, contact resistance ∼30 kΩ, peak transconductance 1.2 μS/μm, and high-fidelity ac operation at frequencies up to 10 kHz. The device consists of a GaAs nanowire with an undoped core and heavily Be-doped shell. We carefully etch back the nanowire at the gate locations to obtain Schottky-barrier insulated gates while leaving the doped shell intact at the contacts to obtain low contact resistance. Our device opens a path to all-GaAs nanowire MESFET complementary circuits with simplified fabrication and improved performance.