The present invention provides a thermally curable resin sheet for sealing a semiconductor chip having excellent reliability and storability while being reduced in warpage deformation due to the volume shrinkage of the thermally curable resin sheet, and a method for manufacturing a semiconductor package. The present invention relates to a thermally curable resin sheet for sealing a semiconductor chip, wherein an activation energy (Ea) satisfies the following formula (1), a glass transition temperature of a product thermally cured at 150° C. for 1 hour is 125° C. or higher, and a thermal expansion coefficient α [ppm/K] of the thermally cured product at the glass transition temperature or lower and a storage modulus E′ [GPa] at 25° C. of the thermally cured product satisfy the following formula (2): 30≦Ea≦120 [kJ/mol]  (1); and 10,000≦α×E′≦300,000 [Pa/K]  (2).