The present work reports on the, first of its kind, synthesis of potassium co-doped aluminum-doped ZnO nanoparticles (NPs) through thermo-sol gelation process. The co-doping of K on Al–ZnO NPs is found having a significant influence on the band gap narrowing of ZnO, to an extent that the entire light absorption is shifted towards the visible-light spectrum. The synthesized NPs are characterized using, XRD, TEM, SEM, EDX, UV–Vis–DRS, BET surface area analysis, TG/DTA, PL, and ICP–OES. The band gap energy levels for bare ZnO, Al-doped ZnO, and K co-doped Al–ZnO NPs are 3.26, 3.03 and 2.88 eV, respectively. A systematic study has been carried to evaluate the photocatalytic properties of K co-doped Al–ZnO photocatalyst through Direct Green 6 (a textile dye) degradation studies, and has been compared with well-known Al–ZnO NPs. The effect of various operational parameters such as, co-dopant/dopant stoichiometry, solution pH, catalyst dosage, dye concentration, kinetics, photosensitizers, light intensity, and matrix tolerance, on the dye degradation, has been systematically evaluated. The analytical data reveal that under optimized conditions, the K co-doped Al–ZnO photocatalyst can induce complete dye degradation within 0.5 h under visible light. A comprehensive mechanism has been proposed on the photocatalytic activity of K–Al–ZnO NPs. © 2016, Springer Science+Business Media New York.