In the paper, the structure and static and dynamic mechanical properties of ultrafine-grained A5083 alloy (Al-Mg-Mn) produced by high-pressure torsion (HPT) are reported. The static yield stress and tensile strength were determined in tensile tests at a strain rate of ~ 10⁻³ s⁻¹, and the dynamic yield stress and spall strength were calculated from free-surface velocity histories obtained during shock-wave loading at a strain rate of 10⁵ s⁻¹. The HPT technique provides strong grain refinement. The average grain size of the alloy after HPT is 100-180 nm and depends on the accumulated true strain. HPT significantly improves the static strength properties of the alloy. The static yield stress is increased by 360-390% and the static ultimate tensile strength by 166-182%. It is shown that the dynamic yield stress improved by 168-181%, while the dynamic spall strength was not improved by HPT. Moreover, the nanostructured alloy with a grain size of ~ 100 nm demonstrates the lowest spall strength.