A high-strength steel sheet comprising: a chemical composition consisting of, in mass%, C: 0.030 % or more and 0.250 % or less, Si: 0.01 % or more and 3.00 % or less, Mn: more than 4.20 % and 6.00 % or less, P: 0.001 % or more and 0.100 % or less, S: 0.0001 % or more and 0.0200 % or less, N: 0.0005 % or more and 0.0100 % or less, and Ti: 0.003 % or more and 0.200 % or less, and optionally one or more of Al: 0.01 % or more and 2.00 % or less, Nb: 0.005 % or more and 0.200 % or less, B: 0.0003 % or more and 0.0050 % or less, Ni: 0.005 % or more and 1.000 % or less, Cr: 0.005 % or more and 1.000 % or less, V: 0.005 % or more and 0.500 % or less, Mo: 0.005 % or more and 1.000 % or less, Cu: 0.005 % or more and 1.000 % or less, Sn: 0.002 % or more and 0.200 % or less, Sb: 0.002 % or more and 0.200 % or less, Ta: 0.001 % or more and 0.010 % or less, Ca: 0.0005 % or more and 0.0050 % or less, Mg: 0.0005 % or more and 0.0050 % or less, and REM: 0.0005 % or more and 0.0050 % or less and the balance consisting of Fe and incidental impurities; and a steel microstructure that contains, in area ratio, 15 % or more and 55 % or less of polygonal ferrite and 15 % or more and 30 % or less of martensite, and that contains, in volume fraction, 12 % or more of retained austenite, wherein the polygonal ferrite has a mean grain size of 4 μm or less, the martensite has a mean grain size of 2 μm or less, the retained austenite has a mean grain size of 2 μm or less, and the polygonal ferrite, the martensite, and the retained austenite each have a mean grain aspect ratio of 2.0 or less, and wherein a value obtained by dividing an Mn content in the retained austenite in mass% by an Mn content in the polygonal ferrite in mass% equals 2.0 or more. a chemical composition consisting of, in mass%, C: 0.030 % or more and 0.250 % or less, Si: 0.01 % or more and 3.00 % or less, Mn: more than 4.20 % and 6.00 % or less, P: 0.001 % or more and 0.100 % or less, S: 0.0001 % or more and 0.0200 % or less, N: 0.0005 % or more and 0.0100 % or less, and Ti: 0.003 % or more and 0.200 % or less, and optionally one or more of Al: 0.01 % or more and 2.00 % or less, Nb: 0.005 % or more and 0.200 % or less, B: 0.0003 % or more and 0.0050 % or less, Ni: 0.005 % or more and 1.000 % or less, Cr: 0.005 % or more and 1.000 % or less, V: 0.005 % or more and 0.500 % or less, Mo: 0.005 % or more and 1.000 % or less, Cu: 0.005 % or more and 1.000 % or less, Sn: 0.002 % or more and 0.200 % or less, Sb: 0.002 % or more and 0.200 % or less, Ta: 0.001 % or more and 0.010 % or less, Ca: 0.0005 % or more and 0.0050 % or less, Mg: 0.0005 % or more and 0.0050 % or less, and REM: 0.0005 % or more and 0.0050 % or less and the balance consisting of Fe and incidental impurities; and a steel microstructure that contains, in area ratio, 15 % or more and 55 % or less of polygonal ferrite and 15 % or more and 30 % or less of martensite, and that contains, in volume fraction, 12 % or more of retained austenite, wherein the polygonal ferrite has a mean grain size of 4 μm or less, the martensite has a mean grain size of 2 μm or less, the retained austenite has a mean grain size of 2 μm or less, and the polygonal ferrite, the martensite, and the retained austenite each have a mean grain aspect ratio of 2.0 or less, and wherein a value obtained by dividing an Mn content in the retained austenite in mass% by an Mn content in the polygonal ferrite in mass% equals 2.0 or more.