A novel alkali-activated magnesium slag-based cementitious nanocomposite (AMSCN) was synthesized by co-loaded NiO and CuO, and employed as a photocatalyst for the degradation of methyl orange dye. The FESEM images reveal that the AMSCN has a nanostructural morphology with average size of about 30 nm. The XRD patterns show that the AMSCN is principally composed of mineralogical phase of calcium silicate hydrate (C-S-H). NiO active species are uniformly dispersed in the form of amorphous phase and CuO emerged nanophase with mean size of 15 nm on the surface of AMSCN. The 10(NiO+CuO)/AMSCN nanocomposite exhibits the highest photocatalytic degradation rate (93%) due to the synergistic effect between the AMSCN matrix and active species of NiO and CuO. The kinetics analysis indicates that the photocatalytic degradation of methyl orange follows the first order reaction kinetics