A curable silicone composition comprising: (A) an organopolysiloxane having at least two alkenyl groups in a molecule; (B) an organohydrogenpolysiloxane having at least two silicon-bonded hydrogen atoms in a molecule; (C) an adhesion promoter represented by the average unit formula; and (D) a hydrosilylation reaction catalyst. Provided is a curable silicone composition for forming a transparent cured product having high adhesion to a substrate, and an optical semiconductor device having excellent reliability made using the composition.