In this study, the celestine (natural strontium sulfate) is first converted to strontium carbonate (SrCO3). Then, magnetic strontium ferrite nanocomposite was produced from the reaction of obtained SrCO3(in HNO3) and Fe(NO3)3solution. Biosynthesized Ag nanoparticles (NPs) supported on SrFe2O4were prepared by the aqueous horehound (Marrubium vulgareL.) leaf extract. The synthesized nanocomposites were analyzed using different characterization techniques, including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), field emission scanning electron microscope (FESEM), and X-ray spectroscopy (EDX). The FESEM analysis of Ag/SrFe2O4nanocomposite confirms the formation of spherical Ag NPs dispersed on the surface of strontium ferrite with an average particle size below 50 nm. The reduction/decolorization time of 10 ppm methyl orange (MO) solution in the presence of 10.6 × 10−3M sodium borohydride (NaBH4) solution and 9 mg of Ag/SrFe2O4catalyst with good activity and reusability was 2.5 min.