Individual alkanethiol-functionalized gold nanorods act as morphological seeds that specifically template the growth and direction of large, uniform, cylindrical-phase domains from a polystyrene-block-polyisoprene (PS-b-PI) copolymer solution. Adding less than 0.0001 vol % nanorods switched the preference of domain growth from predominantly spherulitic to almost exclusively single crystalline, with the PI cylinders uniformly aligned in the same direction as each seeding nanorod. Polarized and dark-field optical microscopy on individual domains and small-angle X-ray scattering experiments on bulk material confirmed this specific seeding effect. By contrast, we find that spherical gold nanoparticles and other materials that are commonly used as heterogeneous nucleants in crystallization (e.g., carbon black, talc, and calcium carbonate) failed to promote single-crystalline domain growth in the same material. Mesoscale dynamic modeling of nanorod-copolymer mixtures illustrates these observations in terms of a specific interaction between the surface of the nanorods and the polyisoprene block, which orients the self-assembling polyisoprene cylinders in the nascent domain nucleus.