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Molecular-scale components are expected to be central to the realization of nanoscale electronic devices^,,. Although molecular-scale switching has been reported in atomic quantum point contacts^,,, single-molecule junctions provide the additional flexibility of tuning the on/off conductance states through molecular design. To date, switching in single-molecule junctions has been attributed to changes in the conformation or charge state of the molecule^,,,,,. Here, we demonstrate reversible binary switching in a single-molecule junction by mechanical control of the metal–molecule contact geometry. We show that 4,4'-bipyridine–gold single-molecule junctions can be reversibly switched between two conductance states through repeated junction elongation and compression. Using first-principles calculations, we attribute the different measured conductance states to distinct contact geometries at the flexible but stable …