The size, shape, composition, and crystalline structures of noble metal nanoparticles are the key parameters in determining their electrocatalytic performance. Here, we report on a robust chemical-tethering approach to immobilizing gold nanoparticles onto transparent indium tin oxide (ITO) glass electrode surfaces to systematically investigate their size- and shape-dependent electrocatalysis toward a methanol oxidation reaction (MOR) and an oxygen reduction reaction (ORR). Monodisperse 20 nm nanospheres (NS20s), 45 nm nanospheres (NS45s), and 20 nm x 63 nm nanorods (Nits), which could be chemically tethered to ITO-surface-forming submonolayers without any aggregation, were synthesized. These nanoparticle-modified ITO electrodes exhibited strong electrocatalytic activities toward MOR and ORR, but their mass current densities were highly dependent on the particle sizes and shapes. For particles with similar shapes, the size determined the mass current densities: smaller particle sizes led to greater catalytic current densities per unit mass because of the greater surface-to-volume ratio (NS20s > NS45s). For particles with comparable sizes, the shape or crystalline structure governed the selectivity of the electrocatalytic reactions: NS45 exhibited a higher mass current density in MOR than did NRs because its dominant (111) facets were exposed, whereas NRs exhibited a higher mass current density in ORR because its dominant (100) facets were exposed.