FOR at least the past two billion years, volcanism on Mars has been restricted to the Tharsis region(1). In addition; most tectonic activity on Mars(2), together with the long-wavelength topography and the non-hydrostatic gravity field(3), are strongly correlated with Tharsis, implying a close connection with deep mantle processes. These observations have motivated suggestions(4) that the thermal convection in the martian mantle is very different from that in the Earth’s mantle, being dominated by a single large upwelling under Tharsis. Two-dimensional convection modelling has shown that the presence of an endothermic phase boundary in the lowermost mantle of a planet has a strong influence on convection, suppressing all but one or two upwellings(5). Recent experiments(6) indicate that such a boundary may indeed exist on Mars. Here we investigate the convective evolution of the martian mantle using a three-dimensional model which incorporates an endothermic phase transition close to the core-mantle boundary. We find that a single-plume pattern of convection gradually develops, and we show that this is consistent with the distribution of volcanism, the shape of the gravity field, and the gross tectonic stress pattern of Mars.