Ras proteins occupy dynamic plasma membrane nanodomains called nanoclusters. The significance of this spatial organization is unknown. Here we show, using in silico and in vivo analyses of mitogen-activated protein (MAP) kinase signalling, that Ras nanoclusters operate as sensitive switches, converting graded ligand inputs into fixed outputs of activated extracellular signal-regulated kinase (ERK). By generating Ras nanoclusters in direct proportion to ligand input, cells build an analogue-digital-analogue circuit relay that transmits a signal across the plasma membrane with high fidelity. Signal transmission is completely dependent on Ras spatial organization and fails if nanoclustering is abrogated. A requirement for high-fidelity signalling may explain the non-random distribution of other plasma membrane signalling complexes.