In this study, the microstructure control and photocatalytic performance optimization of bismuth vanadate (BiVO₄) were investigated by adjusting the hydrothermal reaction time. BiVO₄ nanostructures were synthesized via the hydrothermal method at 200 °C for 4 h, 6 h, and 8 h, and the effect of synthesis time on their microstructural features, optical properties, and photocatalytic activity was evaluated. X-ray diffraction (XRD) results confirmed the formation of a pure monoclinic scheelite BiVO₄ phase in all samples. XRD Analysis using the Debye–Scherrer, Williamson–Hall, and modified Scherrer methods indicated that with increasing hydrothermal time, the crystallite size gradually increased while lattice strain, defect density, and structural disorders decreased, reflecting enhanced crystallinity and structural evolution. Field-emission scanning electron microscopy (FE-SEM) images confirmed the formation of aggregated nanostructures with gradual particle growth and improved morphological uniformity at longer reaction times. Optical property analysis using UV–Vis diffuse reflectance spectroscopy (DRS) showed that increasing the synthesis time led to enhanced visible light absorption and a reduction in the band gap energy from 2.83 eV to 2.38 eV. The photocatalytic performance of the samples was evaluated in methylene blue degradation under visible light irradiation, and the sample synthesized for 8 h exhibited the highest degradation efficiency (approximately 84 % in 120 min) and the maximum apparent rate constant according to the pseudo-first-order kinetic model. This enhancement in photocatalytic performance was attributed to the synergistic effect of improved microstructure, reduced crystal defects, and increased visible light absorption.