Photocatalytic degradation of organic pollutants is a green and effective route of wastewater treatment. Zinc oxide was initially used for this purpose; however, calcined zinc/chromium layered double hydroxide (ZnCr-LDO) and cadmium sulfide quantum dots (CdSQDs)-based nanocomposites proved as superior alternatives. Herein, we report a green sonochemical method for the sol–gel fabrication of novel CdSQDs@ZnCr-LDO/ZnO ternary nanocomposite that exhibited exceptional photocatalytic activity for the degradation of rhodamine B dye (RhB), in wastewaters, under UV-A-irradiation. The features of the ternary nanocomposite were investigated using various physicochemical techniques, including XRD, SEM, TEM, EDX, XPS, BET, zeta potential, DRS, and PL measurements. The RhB dye % removal was 38.02, 40.2, and 98% using pristine ZnO, ZnCr-LDO and the ternary CdSQDs@ZnCr-LDO/ZnO-based nanomaterials, respectively, reflecting the superior ternary nanocomposite's photocatalytic activity that made it an excellent competitor to commonly reported photocatalysts. Additionally, an investigation was carried out to determine the key reactive species in the photocatalytic degradation of RhB, considering both scavenger's type and concentration. The prevailing mechanism was found to be the reductive photodegradation pathway. Furthermore, several models were utilized to describe the kinetics of photocatalytic performance of the ternary nanocomposite and a typical Z-scheme type-II photocatalytic heterojunction mechanism was inferred.