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Ultrafine grained (UFG) alloys suffer from limited strain hardening hence reduced ductility. Incorporation of deformation twinning can overcome this limitation. An Al_0.3CoCrFeNi high-entropy alloy was thermo-mechanically processed to yield a UFG triplex microstructure (d_avg: 0.71 ± 0.35 μm) with FCC, and hard B2 and sigma phases. The annealed material exhibited an exceptional strength-ductility combination with ultimate tensile strength (UTS) of ~1.1 GPa and elongation of ~25%. Furthermore, the alloy showed excellent fatigue resistance with an endurance limit of ~0.43 UTS. Remarkable mechanical properties are attributed to the formation of extensive deformation nanotwins and increased dislocation accumulation capability.