Growing environmental concerns have driven the switch from lead-containing dielectric perovskite ceramics to lead-free alternatives such as silver niobate tantalate [Ag(Nb1−xTax)O3], where tantalum (Ta) substitution for niobium (Nb) enhances energy-storage density. Thin film deposition presents a promising way for fabricating these materials for use in capacitors. In this study, Ag(Nb1−xTax)O3 (0 ≤ x ≤ 0.5) thin films are synthesized via combinatorial reactive dc magnetron sputtering from metallic targets. The chemical and phase compositions of the films are comprehensively analyzed using scanning electron microscopy coupled with energy dispersive x-ray spectroscopy, elastic recoil detection analysis, Rutherford backscattering spectrometry, x-ray diffraction, Raman spectroscopy, and x-ray photoelectron spectroscopy. The findings demonstrate that reactive dc magnetron sputtering is a feasible technique for producing complex perovskite oxide thin films with customized chemical composition and microstructure. By enhancing the understanding of the Ag(Nb1−xTax)O3 material system, this study aims to contribute to the development of environmentally benign high-performance dielectrics that could replace lead-based ceramics in energy-storage applications.