An epoxy-functionalized polyorganosiloxane toughener providing toughening properties for thermosetting resins is described. The epoxy-functionalized polyorganosiloxane toughener is an epoxy-functionalized siloxane polyether with a siloxane backbone, at least one epoxy-containing alkyl chain, and at least one alkoxyl polyether chain, each said at least one epoxy-containing alkyl chain and at least one alkoxyl polyether chain grafted to the siloxane backbone. Also disclosed is a thermosetting resin-based composition comprising the epoxy-functionalized polyorganosiloxane toughener of claim 1 and at least one of an epoxy resin, a hardener, and an accelerator.