It is an object of the present invention to provide a thermal transfer sheet which has a high transfer sensitivity in thermal transfer printing to obtain a high density print, has a high sharpness of thermal transfer images, can prevent an abnormal transfer in printing even after being stored at high temperature and high humidity, and can provide a sufficiently satisfactory printed matter.The above object is achieved by a thermal transfer sheet comprising: a substrate; a heat resistant slip layer provided on one side of the substrate; an undercoat layer and a dye layer provided in that order on the other side of the substrate, wherein the undercoat layer is formed by applying and drying a coating liquid which contains, as main components, a water soluble self cross-linking resin and colloidal inorganic pigment ultrafine particles, and cross-linking polymerizing the water soluble self cross-linking resin, or wherein the undercoat layer is formed by using colloidal inorganic pigment ultrafine particles and a copolymer resin of vinyl pyrrolidone and vinyl acetate as main components.