In this study, a facile synthesis of porous carbon and its Fe3O4 nanocomposite were synthesized via carbonization of red water and co-precipitation reaction, respectively. Then, an aqueous extract of Caesalpinia gilliesii flower was used for reduction of the Ag+ ions to Ag nanoparticles (Ag NPs) and their stabilization on the surface of carbon supports. The formation of the Ag NPs occurs at room temperature within a few minutes. The synthesized porous carbon and its nanocomposites were characterized by FTIR, Raman, XRD, FESEM, EDS, elemental mapping, TEM, BET, and VSM techniques. According to the FESEM and TEM images, the average size of the Ag NPs on the surface of porous carbon and C/Fe3O4 was below 35 nm. The magnetically recoverable Ag/Fe3O4/C nanocomposite demonstrated favorable catalytic activity on the reduction of methyl orange (MO) and 4-nitrophenol (4-NP). In addition, the catalyst can be reused at least 3 times without considerable loss in its activity.