The present invention relates to a curable silicone composition comprising: (A) a diorganopolysiloxane having at least two alkenyl groups in a molecule, (B) at least two types of resinous organopolysiloxanes having different mass average molecular weights, (C) an organopolysiloxane having at least two silicon-bonded hydrogen atoms in a molecule, and (D) a hydrosilylation reaction catalyst. The curable silicone composition enables the production of an optical semiconductor device in which the marked viscosity elevation of the resulting composition can be inhibited, in which the fluidity and packing properties are outstanding and that has outstanding gas barrier properties when used as a sealant, and in which the device has outstanding initial optical output efficiency even when organopolysiloxane resin is blended in order to form cured product with moderate hardness and strength.