Abstract: Visible-light–active semiconductor nanoparticles with tunable structural and defect-related properties represent a versatile and promising class of materials for photocatalytic and photoelectrochemical systems, enabling enhanced performance and flexible control over functional properties. In this work, SnO2 spherical nanoparticles were synthesized via a reproducible co-precipitation method with controlled Al3+ and Fe3+ doping (11 and 33 mol %), enabling systematic modulation of lattice parameters, defect density, and oxygen vacancy content. Comprehensive characterization using XRD, FTIR, BET, HRTEM, SAED, EDX, XPS, Raman spectroscopy, UV-Vis absorption, and DLS, combined with DFT calculations, provided insight into dopant sites in the crystal lattice. The nanoparticles exhibited pronounced visible-light photocatalytic activity against methylene blue dye. Electrodes modified with the optimal sample demonstrated a photoinduced electrochemical response toward ascorbic acid over a wide concentration range. Notably, the same electrode produced distinct signals under different illumination conditions, indicating that the electrochemical response can be dynamically tuned by light. These results highlight a strategy for designing materials that combine visible-light photocatalysis with light-controlled electrochemical response, enabling new approaches to signal modulation in electrochemical systems.