Composite particles consisting of elastic silicone and glassy poly(methyl methacrylate) (PMMA) were prepared. Seeded polymerization of methyl methacrylate (MMA) was carried out in the presence of silicone elastic particles, which were prepared via a radical-mediated thiol–ene click reaction between thiol-terminated silicone oil and a trifunctional allyl compound in a suspension system. The obtained particles exhibited a sea-island structure comprising small PMMA domains and a silicone matrix, and the size of the PMMA domains could be controlled via the polymerization temperature. Moreover, core–shell composite particles consisting of a silicone core and PMMA shell were successfully prepared using highly cross-linked silicone seed particles. Interestingly, the morphology of the composite particles could be alternated between sea-island and core–shell structures by controlling the annealing temperature. Microcompression tests indicated that, the hardness of the composite particles also can be changed: the hardness of sea-island composite particles was similar to the silicone particles, that of the core–shell particles was more than seven times greater than that of the silicone particles.