The ability to design nanomaterial characteristics requires exact control of particle structure due to the sensitive relationship between the material properties and the particle size and morphology. Borrowing nature's approach where biomineralization is controlled by macromolecules, synthetic polymeric matrices are used to guide the formation of stable, monodisperse iron oxide nanoparticles uniformly distributed in the polymeric matrix. Particle size, size distribution, and morphology are determined by the polymer-particle interactions and are independent of the chain molecular weight: γ-Fe2O3 particles formed in strongly interacting polymeric media are small (∼ 10-20 nm) and pyramidal in shape, while those formed in weakly interacting media are larger (∼ 40-60 nm) and spherical. This synthesis method can be easily extended to a variety of inorganic nanoparticle chemistries, thereby enabling exact control over material properties.