Titanium dioxide nanoparticles (TiO2 NPs) are widely employed as semiconductor materials in dye-sensitized solar cells (DSSCs) photoanodes. However, conventional chemical and physical synthesis methods to synthesize TiO2 NPs raise environmental concerns. This study presents an eco-friendly synthesis route for TiO2 NPs using gambier (Uncaria gambir) leaf extract as a natural capping agent coupled with the use of a chelating agent. The phytochemical composition of the extract was characterized using liquid chromatography-mass spectrometry (LC-MS) and Fourier transform infrared spectroscopy (FTIR). The optical, structural, and morphological properties of the synthesized NPs were investigated using UV–Vis diffuse reflectance spectroscopy (UV-DRS), X-ray diffraction (XRD), field emission electron microscopy coupled with energy dispersive X-ray spectroscopy (FESEM/EDS), Raman spectroscopy, and high-resolution transmission electron microscopy coupled with selected area diffraction (HRTEM/SAED). The green-synthesized TiO2 NPs exhibited a reduced bandgap energy of 3.04 eV compared to 3.2 eV for commercial and ethanol-synthesized counterparts. While acetylacetone as a chelating agent showed mixed effects, its 30 % concentration yielded a bandgap of 3.07 eV. Remarkably, TiO2 NPs synthesized with gambier extract and 50 % acetylacetone achieved the highest DSSC efficiency of 4.40 %, significantly outperforming commercial TiO2 (2.83 %). These findings demonstrate the potential of plant-based synthesis in advancing sustainable nanomaterials for photovoltaic applications. © 2025 The Authors