Provided are a high-strength steel sheet having a specified chemical composition, in which a Mn-segregation degree in a region within 100 μm from a surface thereof in a thickness direction is 1.5 or less, in a plane parallel to the surface of the steel sheet in a region within 100 μm from the surface of the steel sheet in the thickness direction, the number of oxide-based inclusion grains having a grain long diameter of 5 μm or more is 1000 or less/100 mm2, a proportion of the number of oxide-based inclusion grains having a chemical composition containing alumina of 50 mass % or more, silica of 20 mass % or less, and calcia of 40 mass % or less to the total number of oxide-based inclusions having a grain long diameter of 5 μm or more is 80% or more, a specified metallographic structure, and a TS of 980 MPa or more, a high-strength galvanized steel sheet, and a manufacturing method thereof.