Abstract: Provided are: a thermosetting resin composition from which a molded body exhibiting little change in optical characteristics when exposed to high temperatures can be obtained, and which can be applied to various packaging processes; an organosilicon compound from which the composition can be achieved; and a molded body and an optical semiconductor device which are obtained using such a thermosetting resin composition. The thermosetting resin composition includes: a compound that is selected from among organosilicon compounds (A) represented by Formula (1-1) or Formula (1-2); an organosilicon compound (B) that has a plurality of crosslinkable groups in its molecule and is other than the organosilicon compounds (A); and a hydrosilylation catalyst (C), in which the organosilicon compound (B) contains a compound that is crosslinkable with the organosilicon compounds (A).