A silicone resin composition that exhibits low gas permeability and is suitable for encapsulating optical semiconductors. The composition includes: (A) an organopolysiloxane having a specific structure containing two or more alkenyl groups, (B) an organohydrogenpolysiloxane composed of two organohydrogenpolysiloxanes having specific structures, in which the mass ratio between the two organohydrogenpolysiloxanes is within a range from 10:90 to 90:10, in an amount that provides 0.4 to 4.0 mols of silicon atom-bonded hydrogen atoms within the component (B) per 1 mol of alkenyl groups within the component (A), and (C) an addition reaction catalyst.