A CoCrFeMnNi high-entropy alloy (HEA) showed elongation to failure ∼320% at T = 1023 K and a strain rate (Formula presented.). Strain hardening and texture weakening occurred at low (Formula presented.), whereas flow softening and texture strengthening were observed at high (Formula presented.) experiments. The strain rate sensitivity (m) decreased from 0.5 to ∼0.25 with increasing (Formula presented.). Deformation with m∼0.5 and deformation-enhanced grain growth at low (Formula presented.) indicated superplasticity associated with grain boundary sliding. The grain boundary diffusion coefficient diminished by a factor of ∼4 in the HEA. Concurrent nucleation, growth and cavity interlinkage caused premature failure compared with conventional superplastic alloys. © 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.