Scintillator materials are used as detectors in the ray imaging techniques for medical diagnosis. Because the ideal medical scintillator material does not exist, many efforts are being made to find new materials that satisfy a greater number of properties. Here, the synthesis conditions of Pr:KGd(PO3)4 nanocrystals by the modified Pechini method are optimized to obtain a single crystalline phase of those that form the polymorphism of KGd(PO3)4. The interest lies in the type III phase because less quenching by Pr3+ concentration is expected. By performing transmittance measurements and because of the wide transparency window of the type III KGd(PO3)4 host, the 3H4 → 5d1 absorption transition of Pr3+ has been observed in the vacuum ultraviolet spectral range. After creating electron-hole pairs in the host due to the excitation of the material by X-ray radiation, the bands corresponding to the 5d1 → 3H4, 3H5, 3H6 and 5d1 → 3F3, 3F4, 1G4 transitions of Pr3+ have been observed in the near-visible spectral range, being these 5d → 4f transitions interesting for scintillation applications. Therefore, the type III Pr:KGd(PO3)4 nanocrystals allow the conversion from high-energy radiation to visible or near-visible light.