A high-strength steel sheet having a tensile strength of 590 MPa or more and a yield ratio of 0.63 or less, the high-strength steel sheet having a chemical composition comprising, in mass%, C: 0.06% or greater and 0.120% or less, Si: 0.3% or greater and 0.7% or less, Mn: 1.6% or greater and 2.2% or less, P: 0.05% or less, S: 0.0050% or less, Al: 0.01% or greater and 0.20% or less, and N: 0.010% or less, optionally one or more selected from Cr: 0.01% or greater and 0.20% or less, Mo: 0.01% or greater and less than 0.15%, and V: 0.001% or greater and 0.05% or less; and optionally one or more selected from Groups A to C described below: Group A: one or two selected from Nb: 0.001% or greater and 0.02% or less and Ti: 0.001% or greater and 0.02% or less, Group B: one or two selected from Cu: 0.001% or greater and 0.20% or less and Ni: 0.001% or greater and 0.10% or less, and Group C: B: 0.0001% or greater and 0.002% or less; with a balance of Fe and incidental impurities, the high-strength steel sheet having a microstructure in which ferrite is present as a major phase in an area fraction of 50 to 100% relative to the area of an entirety of the microstructure, and martensite is present in an area fraction of 10% or greater and less than 50% relative to an area of an entirety of the microstructure, wherein the martensite has an average grain diameter of 3.0 μm or less, in an entirety of the martensite, a proportion of martensite having an aspect ratio of 3 or less is 60% or greater in terms of an area fraction, and the martensite having an aspect ratio of 3 or less has a carbon concentration of 0.30% or greater and 0.90% or less in mass%, whereby the microstructure and the aspect ratio are determined according to the methods disclosed in the description. the high-strength steel sheet having a chemical composition comprising, in mass%, C: 0.06% or greater and 0.120% or less, Si: 0.3% or greater and 0.7% or less, Mn: 1.6% or greater and 2.2% or less, P: 0.05% or less, S: 0.0050% or less, Al: 0.01% or greater and 0.20% or less, and N: 0.010% or less, optionally one or more selected from Cr: 0.01% or greater and 0.20% or less, Mo: 0.01% or greater and less than 0.15%, and V: 0.001% or greater and 0.05% or less; and optionally one or more selected from Groups A to C described below: Group A: one or two selected from Nb: 0.001% or greater and 0.02% or less and Ti: 0.001% or greater and 0.02% or less, Group B: one or two selected from Cu: 0.001% or greater and 0.20% or less and Ni: 0.001% or greater and 0.10% or less, and Group C: B: 0.0001% or greater and 0.002% or less; with a balance of Fe and incidental impurities, the high-strength steel sheet having a microstructure in which ferrite is present as a major phase in an area fraction of 50 to 100% relative to the area of an entirety of the microstructure, and martensite is present in an area fraction of 10% or greater and less than 50% relative to an area of an entirety of the microstructure, wherein the martensite has an average grain diameter of 3.0 μm or less, in an entirety of the martensite, a proportion of martensite having an aspect ratio of 3 or less is 60% or greater in terms of an area fraction, and the martensite having an aspect ratio of 3 or less has a carbon concentration of 0.30% or greater and 0.90% or less in mass%, whereby the microstructure and the aspect ratio are determined according to the methods disclosed in the description. Group A: one or two selected from Nb: 0.001% or greater and 0.02% or less and Ti: 0.001% or greater and 0.02% or less, Group B: one or two selected from Cu: 0.001% or greater and 0.20% or less and Ni: 0.001% or greater and 0.10% or less, and Group C: B: 0.0001% or greater and 0.002% or less; with a balance of Fe and incidental impurities, the high-strength steel sheet having a microstructure in which ferrite is present as a major phase in an area fraction of 50 to 100% relative to the area of an entirety of the microstructure, and martensite is present in an area fraction of 10% or greater and less than 50% relative to an area of an entirety of the microstructure, wherein the martensite has an average grain diameter of 3.0 μm or less, in an entirety of the martensite, a proportion of martensite having an aspect ratio of 3 or less is 60% or greater in terms of an area fraction, and the martensite having an aspect ratio of 3 or less has a carbon concentration of 0.30% or greater and 0.90% or less in mass%, whereby the microstructure and the aspect ratio are determined according to the methods disclosed in the description.