Magnetite (Fe3O4) NPs as well as bacterial cellulose (BC)/Fe3O4 and graphene (Gr)/BC/Fe3O4 nanocomposites (NCs) were synthesized by a hydrothermal method and characterized by spectroscopic, crystallographic and microscopic characterization techniques. Their antibacterial activities were subsequently tested against ESBL-producing Escherichia coli and Proteus mirabilis. Gr/BC/Fe3O4 NCs showed 100% antibacterial activity, whereas BC/Fe3O4 NCs and Fe3O4 NPs showed values of 97.3% and 94.6% as well as 96.6% and 94.3%, respectively, for E. coli and P. mirabilis at a concentration of 100 μg/mL. The minimum inhibitory concentration of Gr/BC/Fe3O4 NCs against E. coli and P. mirabilis were found to be 60 and 70 μg/mL, respectively. The photocatalytic degradation efficiencies of Fe3O4 NPs as well as BC/Fe3O4 and Gr/BC/Fe3O4 NCs were quantified by UV–visible spectrophotometry against organic dye molecules including bromothymol blue (BTB), rhodamine B (Rh-B) and methylene blue (MB) upon exposure to visible light. Gr/BC/Fe3O4 NCs showed the fastest (40 min) and 100% degradation efficiency against BTB, whereas 100% of Rh-B and MB were degraded within 50 min and 70 min, respectively. Our results demonstrate the multifunctionality of Gr/BC/Fe3O4 NCs for use as highly efficient antibacterial agent and photocatalyst.