With a purpose of obtaining materials having high specific strength at room temperature and superplastic elongation at elevated temperatures, Mg–Al–Zn ternary alloys with higher contents of alloying elements than the commercial Mg–Al–Zn casting alloys were rapidly solidified by gas atomizing and subsequent splat quenching. The rapidly solidified flakes were consolidated to the P/M materials by hot extrusion at 573 K. The cast ingots of these alloys were also hot-extruded as the reference I/M materials. The obtained P/M materials showed finer dispersion of the second phase particles than the I/M counterparts. The hardness and tensile strength of P/M and I/M materials increased linearly in parallel to each other with increasing Al+Zn content in atomic %. The highest tensile strength of 447 MPa at room temperature was obtained for rapidly solidified Mg–8 mass%Al–12 mass%Zn. The decreases in tensile strength and increases in elongation with rising test temperature were more pronounced in the P/M materials than in the I/M materials. At 573 K, tensile strength of the P/M materials decreased to below 20 MPa and elongation increased to above 100% for a wide range of tensile strain rate. The highest elongation was larger than 900% which was observed at 573 K for the Mg–10 mass%Al–5 mass%Zn P/M material at an initial strain rate of 2 × 10⁻²/s.