High-quality single-crystalline germanium oxide nanowires (GeONWs) have been synthesized by a thermal evaporation method under a vapor–liquid–solid mechanism. The morphology and nanostructure of the as-synthesized GeONWs were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) of the GeONWs showed the presence of substoichiometric oxygen. The electronic structure and local structure of GeONWs were also investigated by X-ray absorption fine structure (XAFS), which showed disorder and degradation of long-range order in the nanowires due to the nanosize in one-dimension and oxygen vacancies in the nanowires. Synchrotron-radiation-based X-ray-excited optical luminescence (XEOL) from GeONWs exhibits strong green light at 2.3 eV (540 nm), which corresponds to the photoluminescence (PL) spectrum of the individual GeONWs excited by laser. The strong emission is attributed to oxygen-related defect states in oxygen vacancies confirmed by X-ray absorption near-edge structure (XANES) and XEOL.