Transition-metal nanoparticles possess unique size-dependent optical, electronic, and catalytic properties on the nanoscale, which differ significantly from their bulk properties. In particular, palladium (Pd) nanoparticles have properties applicable to a wide range of applications in Catalysis and electronics. However, predictable and controllable nanoparticle synthesis remains challenging because of harsh reaction conditions, artifacts from capping agents, and unpredictable growth. Biological supramolecules offer attractive templates for nanoparticle synthesis because of their precise structure and size. In this article. we demonstrate simple, controllable Pd nanoparticle synthesis on surface-assembled viral nanotemplates. Specifically. we exploit precisely spaced thiol functionalities of genetically modified tobacco Mosaic Virus (TMVIcys) for facile surface assembly and readily controllable Pd nanoparticle synthesis via simple electroless deposition Under mild aqueous conditions. Atomic force microscopy (AFM) Studies clearly show tunable surface assembly and Pd nanoparticle formation preferentially on the TMVIcys templates. Grazing incidence small-angle X-ray scattering (GISAXS) further provided all accurate and statistically meaningful route by which to investigate the broad size ranges and uniformity of the Pd nanoparticles formed on TMV templates by simply tuning the reducer concentration. We believe that our viral-templated bottom-up approach to tunable Pd nanoparticle formation combined with the first in-depth characterization via GISAXS represents a major advancement toward exploiting viral templates for facile nanomaterials/device fabrication. We envision that our strategy can be extended to a wide range of applications, including uniform nanostructure and nanocatalyst synthesis.