We describe a method for producing highly monodisperse dumbbell-shaped polymer nanoparticles with dimensions on the order of a few hundred nanometers in extremely high yields. Our technique is based on seeded polymerization, where suspended core-shell particles (linear polystyrene core with polystyrene-co-trimethoxysilylpropylacrylate shell) are used as seeds. When an aqueous suspension of seed particles is mixed with monomer solution, the core-shell particles display dramatic changes in their morphology. Subsequent heating drives the polymerization of monomer, resulting in the formation of dumbbell-shaped particles. The relative sizes of the two Lobes can be controlled by varying the relative volume of the monomer with respect to the seed particle. These particles are well-suited for future studies of the assembly of photonic crystals of anisotropic particles.