The present invention aims to provide an addition-curable silicone resin composition with excellent interfacial adhesion properties, storage stability, and transparency. The present invention also aims to provide an addition-curable silicone resin cured product formed from the addition-curable silicone resin composition and a sealed optical semiconductor element formed by using the addition-curable silicone resin composition. The present invention is an addition-curable silicone resin composition including: an addition-curable silicone resin mixture; and an adhesion-imparting agent, the addition-curable silicone resin mixture having a refractive index of 1.35 to 1.45, the adhesion-imparting agent including a compound that includes a structural unit represented by the formula (1-3) and/or a structural unit represented by the formula (1-4) between a structural unit represented by the formula (1-1) and a structural unit represented by the formula (1-2), the compound having a refractive index of 1.35 to 1.45: wherein R1a's in the formulas (1-1) and (1-2) each independently represent a C1-C18 alkyl group, a cycloalkyl group, an aryl group, or an aralkyl group; R1b's in the formulas (1-3) and (1-4) each independently represent a C1-C18 alkyl group, a cycloalkyl group, an aryl group, or an aralkyl group; m in the formula (1-3) is an integer of 1 to 50; n in the formula (1-4) is an integer of 1 to 1500; and A's in the formulas (1-1) to (1-3) each independently represent a C1-C18 alkyl group, a cycloalkyl group, an aryl group, an aralkyl group, or a group represented by the formula (2), provided that at least one of A's in the formulas (1-1) to (1-3) is a group represented by the formula (2): wherein R2a represents a C1-C8 alkylene group in which a carbon atom other than a carbon atom bonded to a silicon atom is optionally partially substituted with an oxygen atom; R2b's each independently represent a C1-C3 alkylene group; R3's each independently represent a C1-C3 alkylene group; R4's each independently represent a hydrogen atom, a C1-C3 alkyl group, an OH group-containing C1-C3 alkyl group, or a halogeno group; and x is an integer of 0 to 2.