In this article, In2Se3 nanoparticles (NPs) with varying Se/In molar ratios (ranging from 1.3 to 2.1 in increments of 0.2) were synthesized using a simple co-precipitation method. The morphology, crystallization, and chemical stoichiometry of the resulting NPs were analyzed using transmission electron microscopy (TEM), field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) patterns, and energy-dispersive X-ray (EDX) analysis. The structure and chemical bonding of the samples were determined through Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, confirming the XRD results. Additionally, the optical properties were assessed using UV–Vis spectroscopy, revealing a band gap of approximately 1.6 eV for the In2Se3 NPs, which remained consistent across different Se/In molar ratios. The degradation of methylene blue (MB) and malachite green (MG) demonstrated that the samples with Se/In ratios of 1.3 and 1.9 were efficient photocatalytic materials for removing organic contaminants under visible light. The Brunauer-Emmett-Teller (BET) analysis highlighted improved textural parameters for the samples with Se/In ratios of 1.3 and 1.9 compared to the others. Mott-Schottky results indicated that the In2Se3 NPs behaved as n-type semiconductors. Electrochemical impedance spectroscopy (EIS) showed that the sample with a Se/In ratio of 1.9 exhibited lower charge-transfer resistance, which is a key factor in enhancing photocatalytic performance.