The objective of this work was to study the effect of transition metal ion doping (1 wt% of Mn, Fe, Co, Ni, and Cu) in indium oxide (In₂O₃) on its photocatalytic activity to degrade organic dyes, which are considered potential environment pollutants. The transition metal ion-doped In₂O₃ nanocube photocatalyst was prepared via the hydrothermal method. After understanding the thermal behavior of the as-prepared sample (In(OH)₃), it was calcined at 400 °C for 3 h to obtain In₂O₃. The In₂O₃ was systematically investigated via FESEM, X-ray diffraction, Raman spectroscopy and UV-vis absorption analysis. Microstructure analysis by FESEM showed that the In₂O₃ was formed as nanocubes. These nanocubes were formed in a single phase with a cubic crystal structure, while their crystallite size increased from 11 nm to 19 nm when doped with 1 wt% of transition metals, including Mn, Fe, Co, Ni and Cu. The band gap energy for pure In₂O₃ was determined to be 3 eV, and that for the metal ion-doped In₂O₃ showed a slight decrease to the lowest value of 2.94 eV. The photoluminescence (PL) decay lifetime was found to be in the range of 28.56 ns to 33.89 ns. Photocatalytic experiments were conducted for the degradation of methylene blue (MB) dye under sunlight irradiation in the presence of the In₂O₃ nanocubes. Among the five metal ion-doped samples, the Ni ion-doped In₂O₃ photocatalyst exhibited the highest degradation efficiency of 98% in 270 min of sunlight exposure. The high performance of Ni–In₂O₃ is due to its highest PL lifetime of 33.89 ns. The complete route for the degradation of MB dye was revealed by identifying the intermediates.