Using solar energy to reduce extremely noxious Cr(VI) into environmentally friendly Cr(III) presents a promising approach to remedy the contamination of heavy metals. Herein, CdS nanoparticles were dispersed onto the surface of a mesoporous Gd2O3 matrix to construct heterojunction CdS/Gd2O3 nanocomposites for the first time. Cr(VI) photoreduction was employed to determine the photocatalytic abilities of the CdS/Gd2O3 nanocomposites under visible illumination. All CdS/Gd2O3 photocatalysts exhibited superior photocatalytic abilities compared to pristine Gd2O3, and 9 % CdS/Gd2O3 exhibited the highest Cr(VI) photoreduction (100 %) within 30 min compared to pristine Gd2O3 (4 %) and other CdS/Gd2O3 nanocomposites. The Cr(VI) reduction rate over the 9 %CdS/Gd2O3 nanocomposite was 0.1243 min−1 faster than over pristine Gd2O3 (0.0019 min−1). The rate constant of the 9 %CdS/Gd2O3 nanocomposite was 65 times greater than that of pristine Gd2O3. After incorporating CdS onto mesoporous Gd2O3 nanocrystals, the photocurrent of Gd2O3 was significantly enhanced, while its photoluminescence response was substantially reduced, which indicated the enhanced separation efficiency of the charge carrier. This is explained by the synergistic phenomena between CdS and Gd2O3, effective construction p-n heterojunction, and the integration of enhanced charge transfer and effective visible absorption. Furthermore, mesoporous CdS/Gd2O3 nanocomposites presented stable photocatalytic ability during five repeated experiments. The promoted Cr(VI) reduction mechanism over the heterojunction CdS/Gd2O3 photocatalyst was addressed based on an S-scheme carrier transfer process. This research work provides high separation efficiency in charge carriers and the application of CdS/Gd2O3 nanocomposites in the photoreduction of Cr(VI) and oxidation of toxic organic pollutants. Display omitted •Mesoporous heterojunction CdS/Gd2O3 nanocomposites was constructed.•Cr(VI) reduction was utilized to estimate the photocatalytic ability of CdS/Gd2O3.•9 % CdS/Gd2O3 exhibited the highest Cr(VI) reduction (100 %) within 30 min.•The rate of 9 %CdS/Gd2O3 was 65 times greater than that of pristine Gd2O3•There is no significant loss in Cr(VI) reduction after five successive cycles of runs.