A high-strength steel sheet having a tensile strength TS of 780 MPa or more measured in accordance with JIS Z 2241 and comprising: a chemical composition consisting of, in mass%, C: 0.08 % or more and 0.35 % or less, Si: 0.50 % or more and 2.50 % or less, Mn: 1.50 % or more and 3.00 % or less, P: 0.001 % or more and 0.100 % or less, S: 0.0001 % or more and 0.0200 % or less, and N: 0.0005 % or more and 0.0100 % or less, optionally at least one element selected from the group consisting of Al: 0.01 % or more and 1.00 % or less, Ti: 0.005 % or more and 0.100 % or less, Nb: 0.005 % or more and 0.100 % or less, B: 0.0001 % or more and 0.0050 % or less, Cr: 0.05 % or more and 1.00 % or less, Cu: 0.05 % or more and 1.00 % or less, Sb: 0.0020 % or more and 0.2000 % or less, Sn: 0.0020 % or more and 0.2000 % or less, Ta: 0.0010 % or more and 0.1000 % or less, Ca: 0.0003 % or more and 0.0050 % or less, Mg: 0.0003 % or more and 0.0050 % or less, and REM: 0.0003 % or more and 0.0050 % or less, and the balance being Fe and incidental impurities; and a steel microstructure that consists of, by area, 20 % or more and 50 % or less of ferrite, 5 % or more and 25 % or less of bainitic ferrite, 1 % or more and 10 % or less of martensite, and 5 % or more and 15 % or less of tempered martensite, and, by volume, 10 % or more of retained austenite, and by area, 10 % or less of other phases, wherein the retained austenite has a mean grain size of 2 μm or less, a mean Mn content in the retained austenite in mass% is at least 1.2 times the Mn content in the steel sheet in mass%, the retained austenite has a mean free path of 1.2 μm or less, and the tempered martensite has a mean free path of 1.2 μm or less, wherein the mean free path of austenite is calculated by Expression 1 and the mean free path of tempered martensite is calculated by Expression 2 below: L RA = d RA 2 4 π 3 f 1 3 − d RA, with L RA: mean free path of retained austenite (μm) d RA: mean grain size of retained austenite (μm) f: area ratio of retained austenite (%) ÷ 100; L TM = d TM 2 4 π 3 f 1 3 − d TM, with L TM: mean free path of tempered martensite (μm) d TM: mean grain size of tempered martensite (μm) f: area ratio of tempered martensite (%) ÷ 100. a chemical composition consisting of, in mass%, C: 0.08 % or more and 0.35 % or less, Si: 0.50 % or more and 2.50 % or less, Mn: 1.50 % or more and 3.00 % or less, P: 0.001 % or more and 0.100 % or less, S: 0.0001 % or more and 0.0200 % or less, and N: 0.0005 % or more and 0.0100 % or less, optionally at least one element selected from the group consisting of Al: 0.01 % or more and 1.00 % or less, Ti: 0.005 % or more and 0.100 % or less, Nb: 0.005 % or more and 0.100 % or less, B: 0.0001 % or more and 0.0050 % or less, Cr: 0.05 % or more and 1.00 % or less, Cu: 0.05 % or more and 1.00 % or less, Sb: 0.0020 % or more and 0.2000 % or less, Sn: 0.0020 % or more and 0.2000 % or less, Ta: 0.0010 % or more and 0.1000 % or less, Ca: 0.0003 % or more and 0.0050 % or less, Mg: 0.0003 % or more and 0.0050 % or less, and REM: 0.0003 % or more and 0.0050 % or less, and the balance being Fe and incidental impurities; and a steel microstructure that consists of, by area, 20 % or more and 50 % or less of ferrite, 5 % or more and 25 % or less of bainitic ferrite, 1 % or more and 10 % or less of martensite, and 5 % or more and 15 % or less of tempered martensite, and, by volume, 10 % or more of retained austenite, and by area, 10 % or less of other phases, wherein the retained austenite has a mean grain size of 2 μm or less, a mean Mn content in the retained austenite in mass% is at least 1.2 times the Mn content in the steel sheet in mass%, the retained austenite has a mean free path of 1.2 μm or less, and the tempered martensite has a mean free path of 1.2 μm or less, wherein the mean free path of austenite is calculated by Expression 1 and the mean free path of tempered martensite is calculated by Expression 2 below: L RA = d RA 2 4 π 3 f 1 3 − d RA, with L RA: mean free path of retained austenite (μm) d RA: mean grain size of retained austenite (μm) f: area ratio of retained austenite (%) ÷ 100; L TM = d TM 2 4 π 3 f 1 3 − d TM, with L TM: mean free path of tempered martensite (μm) d TM: mean grain size of tempered martensite (μm) f: area ratio of tempered martensite (%) ÷ 100. L RA: mean free path of retained austenite (μm) d RA: mean grain size of retained austenite (μm) f: area ratio of retained austenite (%) ÷ 100; L TM: mean free path of tempered martensite (μm) d TM: mean grain size of tempered martensite (μm) f: area ratio of tempered martensite (%) ÷ 100.