A high-strength steel sheet with excellent ductility and hole-expandability, comprising: a steel sheet with a chemical composition, in mass%, C: 0.05% or more and 0.30% or less, Si: 0.05% or more and 6.00% or less, Mn: 1.50% or more and 10.00% or less, P: 0.000% or more and 0.100% or less, S: 0.000% or more and 0.010% or less, sol. Al: 0.010% or more and 1.000% or less, N: 0.000% or more and 0.010% or less, Ti: 0.000% or more and 0.200% or less, Nb: 0.000% or more and 0.200% or less, V: 0.000% or more and 0.200% or less, Cr: 0.000% or more and 1.000% or less, Mo: 0.000% or more and 1.000% or less, Cu: 0.000% or more and 1.000% or less, Ni: 0.000% or more and 1.000% or less, Ca: 0.0000% or more and 0.0100% or less, Mg: 0.0000% or more and 0.0100% or less, REM: 0.0000% or more and 0.0100% or less, Zr: 0.0000% or more and 0.0100% or less, W: 0.0000% or more and 0.0100% or less, B: 0.0000% or more and 0.0030% or less, and the balance: Fe and inevitable impurities, wherein the steel sheet has a tensile strength of 780 MPa or more, the tensile strength being measured as disclosed in the description,a structure of the steel sheet is composed of, in area ratio, 15% or more and 80% or less of ferrite and 20% or more and 85% or less in total of a hard structure composed of any one of bainite, martensite, or retained austenite, or any combination thereof, and an area ratio of a maximum coupled ferrite region in a region from a position at a depth of 3t/8 from a surface to a position at a depth of t/2, where t is a sheet thickness of the steel sheet, is 80% or more in area ratio to a total ferrite area, and a two-dimensional isoperimetric constant K of the maximum coupled ferrite region is 0.35 or less, wherein the maximum coupled ferrite region has a structure where ferrite grains are continuously coupled in mesh, and the area ratio of the maximum coupled ferrite region is determined as disclosed in the description, and wherein the two-dimensional isoperimetric constant K of the maximum coupled ferrite region is calculated by using the following expression: K = 4 π ∗ S M / L M 2 with L M = peripheral length of maximum coupled ferrite region and S M = area of maximum coupled ferrite region, L M and S M being measured as disclosed in the description. a steel sheet with a chemical composition, in mass%, C: 0.05% or more and 0.30% or less, Si: 0.05% or more and 6.00% or less, Mn: 1.50% or more and 10.00% or less, P: 0.000% or more and 0.100% or less, S: 0.000% or more and 0.010% or less, sol. Al: 0.010% or more and 1.000% or less, N: 0.000% or more and 0.010% or less, Ti: 0.000% or more and 0.200% or less, Nb: 0.000% or more and 0.200% or less, V: 0.000% or more and 0.200% or less, Cr: 0.000% or more and 1.000% or less, Mo: 0.000% or more and 1.000% or less, Cu: 0.000% or more and 1.000% or less, Ni: 0.000% or more and 1.000% or less, Ca: 0.0000% or more and 0.0100% or less, Mg: 0.0000% or more and 0.0100% or less, REM: 0.0000% or more and 0.0100% or less, Zr: 0.0000% or more and 0.0100% or less, W: 0.0000% or more and 0.0100% or less, B: 0.0000% or more and 0.0030% or less, and the balance: Fe and inevitable impurities, wherein the steel sheet has a tensile strength of 780 MPa or more, the tensile strength being measured as disclosed in the description,a structure of the steel sheet is composed of, in area ratio, 15% or more and 80% or less of ferrite and 20% or more and 85% or less in total of a hard structure composed of any one of bainite, martensite, or retained austenite, or any combination thereof, and an area ratio of a maximum coupled ferrite region in a region from a position at a depth of 3t/8 from a surface to a position at a depth of t/2, where t is a sheet thickness of the steel sheet, is 80% or more in area ratio to a total ferrite area, and a two-dimensional isoperimetric constant K of the maximum coupled ferrite region is 0.35 or less, wherein the maximum coupled ferrite region has a structure where ferrite grains are continuously coupled in mesh, and the area ratio of the maximum coupled ferrite region is determined as disclosed in the description, and wherein the two-dimensional isoperimetric constant K of the maximum coupled ferrite region is calculated by using the following expression: K = 4 π ∗ S M / L M 2 with L M = peripheral length of maximum coupled ferrite region and S M = area of maximum coupled ferrite region, L M and S M being measured as disclosed in the description.