Platinum has numerous applications in catalysis, nanoelectronics, and sensing devices. Here we report a method for localized, mask-free deposition of high-purity platinum that employs a combination of room-temperature, direct-write electron beam induced deposition (EBID) using the precursor Pt(PF3)4, and low temperature (≤400 °C) postgrowth annealing in H2O. The annealing treatment removes phosphorus contaminants through a thermally activated pathway involving dissociation of H2O and the subsequent formation of volatile phosphorus oxides and hydrides that desorb during annealing. The resulting Pt is indistinguishable from pure Pt films by wavelength dispersive X-ray spectroscopy (WDS).