A galvannealed steel sheet having a tensile strength of 770 MPa or more comprising: a steel sheet portion containing, by mass %, C: 0.05% to 0.50%, Si: 0.005% to 2.5%, Mn: 0.01% to 3.0%, Al: 0% to 0.5%, Ni: 0% to 2.0%, Cu: 0% to 2.0%, Cr: 0% to 2.0%, Mo: 0% to 2.0%, B: 0% to 0.002%, Ti: 0% to 0.1%, Nb: 0% to 0.1%, V: 0% to 0.1%, REM: 0% to 0.1%, Ca: 0% to 0.1%, P: limited to 0.03% or less, S: limited to 0.02% or less, N: limited to 0.0060% or less, and a remainder of Fe and inevitable impurities; and a plated layer formed on the surface of the steel sheet portion, wherein the plated layer is a galvannealed plated layer, the galvannealed plated layer containing Fe: 5% to 15%, Al: 0.01% to 1% and one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in total of 0% to 3.5% with a remainder of Zn and inevitable impurities, the steel sheet portion has a soft layer that directly adjoins the interface with the plated layer and an inside layer that is other than the soft layer, the thickness D of the soft layer is 0.001% to 5% of the thickness t of the steel sheet portion, wherein in the soft layer a microstructure is present in which the area ratio of ferrite is 95% or more in an average thickness of 0.01 μm to 10 μm on the steel sheet portion side of the interface between the steel sheet portion and the plated layer, and, when the hardness of the soft layer measured by the nano-indentation method is indicated by H1, and the representative hardness of the steel sheet portion measured in the vicinity of a depth of 1/3 of the thickness of the steel sheet by the nano-indentation method is indicated by Ha in a cross section that goes along the thickness direction of the steel sheet portion, H1 is 5% to 75% of Ha, wherein the nano-indentation method is conducted using an atomic force microscope using a diamond indenter having a triangular pyramid shape at the front end, a load on the indenter is adjusted so that the indentation depth in the direction toward the inside layer is 5 nm, the hardness of the steel sheet is measured at locations of a depth of 10 nm pitches along the steel sheet thickness direction, five points for each depth are measured, and an average hardness is obtained for each of the depth locations; and a C concentration in the soft layer is 10% to less than 30% of the C concentration of the entire steel sheet portion. a steel sheet portion containing, by mass %, C: 0.05% to 0.50%, Si: 0.005% to 2.5%, Mn: 0.01% to 3.0%, Al: 0% to 0.5%, Ni: 0% to 2.0%, Cu: 0% to 2.0%, Cr: 0% to 2.0%, Mo: 0% to 2.0%, B: 0% to 0.002%, Ti: 0% to 0.1%, Nb: 0% to 0.1%, V: 0% to 0.1%, REM: 0% to 0.1%, Ca: 0% to 0.1%, P: limited to 0.03% or less, S: limited to 0.02% or less, N: limited to 0.0060% or less, and a remainder of Fe and inevitable impurities; and a plated layer formed on the surface of the steel sheet portion, wherein the plated layer is a galvannealed plated layer, the galvannealed plated layer containing Fe: 5% to 15%, Al: 0.01% to 1% and one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in total of 0% to 3.5% with a remainder of Zn and inevitable impurities, the steel sheet portion has a soft layer that directly adjoins the interface with the plated layer and an inside layer that is other than the soft layer, the thickness D of the soft layer is 0.001% to 5% of the thickness t of the steel sheet portion, wherein in the soft layer a microstructure is present in which the area ratio of ferrite is 95% or more in an average thickness of 0.01 μm to 10 μm on the steel sheet portion side of the interface between the steel sheet portion and the plated layer, and, when the hardness of the soft layer measured by the nano-indentation method is indicated by H1, and the representative hardness of the steel sheet portion measured in the vicinity of a depth of 1/3 of the thickness of the steel sheet by the nano-indentation method is indicated by Ha in a cross section that goes along the thickness direction of the steel sheet portion, H1 is 5% to 75% of Ha, wherein the nano-indentation method is conducted using an atomic force microscope using a diamond indenter having a triangular pyramid shape at the front end, a load on the indenter is adjusted so that the indentation depth in the direction toward the inside layer is 5 nm, the hardness of the steel sheet is measured at locations of a depth of 10 nm pitches along the steel sheet thickness direction, five points for each depth are measured, and an average hardness is obtained for each of the depth locations; and a C concentration in the soft layer is 10% to less than 30% of the C concentration of the entire steel sheet portion.