The present invention aims to provide a condensation-curable silicone resin composition with excellent adhesion properties. The present invention also aims to provide a condensation-curable silicone resin cured product formed from the condensation-curable silicone resin composition and a sealed optical semiconductor element formed by using the condensation-curable silicone resin composition. The present invention is a condensation-curable silicone resin composition including: a condensation-curable silicone resin mixture; and an adhesion-imparting agent, the adhesion-imparting agent including a compound that includes at least one group A, wherein the group A is represented by the formula 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 halogen group; and x is an integer of 0 to 2.