Gold nanoparticles decorated onto titania (anatase) supports were produced through X‐ray radiolysis, at ambient temperature and pressure in a method that departs from traditional routes, providing an outstanding control of Au nanoparticle size and dispersion. Au@TiO₂ nanocomposites (NCs) were fabricated using different absorbed doses in the presence of NaOH, or urea, supported by 5–10 or 10–30 nm titania, to determine their effects on the Au particle size distribution and loading. Au@TiO₂ NCs fabricated at high absorbed doses (>7 kGy) had average Au NP size of 1.1 ± 0.1 nm. Large absorbed doses control Au NP size by generating an abundance of reducing species (e⁻_aq, H•) resulting in an increase of Au seeds. At a lower absorbed dose (0.72 kGy), the ligand affected the Au particle size distribution and a reduction in size by 20% of the Au was observed using NaOH. Au NPs were tethered to titania through oxygen as indicated by X-ray photoelectron spectroscopy analysis. The Au@TiO₂ NCs were tested for their radiosensitizing effect by monitoring decomposition of methylene blue under X-ray irradiation where the decomposition was increased by a factor of 1.8 after 15 min compared with pristine TiO_2.