The planet Mercury rotates three times about its spin axis for every two orbits about the Sun in a 3/2 spin–orbit resonance. This unique state has been explained by an initial rapid prograde rotation, which was then decelerated by tidal torques to the present resonance. When friction at the core–mantle boundary is accounted for, capture into the 3/2 resonance occurs with a probability of only 26%, whereas the most likely outcome is capture into one of the higher-order resonances. Here we use a numerical model of Mercury’s rotational evolution to investigate the consequences of an initial retrograde rotation of Mercury. We find that in this case, the planet would be captured into synchronous rotation, with one hemisphere always facing the Sun, with a probability of 68%. Strong lateral variations in the impact cratering rate would have existed, consistent with the observed distribution of large impact basins. Escape from this highly stable resonance can be initiated by the momentum imparted by large, basin-forming impact events and subsequent capture into the 3/2 resonance is likely. During synchronous rotation, substantial quantities of volatile deposits would have accumulated on the hemisphere facing away from the Sun, potentially explaining the existence of sublimation hollows on Mercury’s surface.