We report a novel atomic layer deposition (ALD) process for SnO₂ that does not require water or other strong oxidizing agents, such as H₂O, O₂ plasma, O₃, or H₂O₂. The development of alternative oxidant-free ALD processes is highly attractive because it enables a gentler and more controllable chemical environment, which is crucial for next-generation nanoscale devices and complex material stacks. In this approach, Sn-based complementary metal precursors are employed: Sn(OtBu)₄ serves as both the tin and oxygen precursor, while SnCl₄ acts as an additional tin source. For this precursor combination, an optimal ALD temperature window of 70–90 °C was established, within which amorphous SnO₂ films are deposited. Post-deposition annealing is subsequently required to develop the desired crystallinity and to adjust the oxygen-vacancy concentration and impurity levels, which ultimately determine the electrical and optical properties of the resulting SnO₂ films and their potential for various applications. To gain mechanistic insight into this oxidant-free SnO₂ growth process, we employ density functional theory (DFT) calculations to investigate the surface reactions during sequential exposure of Sn(OtBu)₄ and SnCl₄ on a SiO₂ substrate. The results reveal key atomic-level processes, including ligand-exchange pathways, oxygen transfer, and surface regeneration, which govern film growth. This combined experimental and theoretical approach provides fundamental understanding of the SnO₂ ALD mechanism and offers guidance for post-deposition optimization of the material.