Nanoscale localisation of the electroforming-free resistive switching (RS) behaviour in polycrystalline GaP thin films has been observed for the first time. A combination of conductive atomic force microscopy and first-order reversal curve current–voltage measurements indicated that the grain boundaries are the preferred sites for the formation of the conductive switching filaments. It is proposed, based on TEM and XPS results, that local electrochemical migration of Ga ions along the grain boundaries plays a critical role in the switching mechanism. In the low-resistance (ON) state, the conduction mechanism was found to be the space-charge-limited current mechanism, while the high-resistance (OFF) state was governed by the Frenkel–Poole mechanism. A high OFF/ON resistance ratio (∼104) and lower power consumption than current RS devices, in addition to the easy integration of GaP with silicon substrates, make these GaP films promising for future applications in future non-volatile resistive random access memory (RRAM).