Provision of a thermally sensitive recording medium made up of an undercoating layer containing a pigment and a binder as main components and a thermally sensitive color-developing layer containing a colorless or pale-colored basic leuco dye and a color-developing agent which develops a color by reacting with said basic leuco dye as main components on a substrate, wherein the undercoating layer contains a water-retention agent and a pigment whose oil-absorbing capacity prescribed by JIS K 5105 is from 80 cc/100 g to 120 cc/100 g as a pigment. Further, the solids concentration of a coating for the undercoating layer is from 25% to 45% and the dynamic water-retention capacity, which is Water retention measured with AA-GWR, is 350 g/m.sup.2 or less.