An ultrafine-grained (UFG) Al-4Mg-1Zr alloy with a grain size of ∼0.7 µm with predominantly high-angle boundaries of 97% was produced by friction stir processing (FSP). The UFG Al-4Mg-1Zr retained submicrometer grains even after static annealing at 425 °C, and exhibited excellent superplasticity at 175-425 °C. High strain rate and low-temperature superplasticity of > 1200% were observed at 1 × 10-2-1 × 10-1 s-1 and 300-350 °C. Even at 425 °C, a superplasticity of 1400% was achieved at 1 s-1. A linear relationship between logover(ε, ̇)opti and T was observed (where over(ε, ̇)opti is the optimum strain rate, and T is the temperature). The analyses on the superplastic data revealed the presence of threshold stress, a stress exponent of 2, an inverse grain size dependence of 2, and an activation energy of 142 kJ mol-1. This indicated that the dominant deformation mechanism was grain boundary sliding, which was controlled by lattice diffusion. Based on this notion, a constitutive equation has been developed. A new superplastic deformation mechanism map for FSP aluminum alloys is proposed.