Herein, semiconductor-magnetic hybrid nanocomposites, poly(vanillin-co-thiophene)/magnetic iron oxide (PVTO-Fe3O4), were successfully synthesized via single-step in situ chemical oxidative polymerization in the presence of different weight percentages of magnetic iron oxide nanoparticles Fe3O4 NPs (3, 6, and 9 wt%). Whereas the Fe3O4 NPs were prepared via an environmentally friendly, modified co-precipitation method, and their physicochemical characteristics were comprehensively analyzed using various analytical techniques. The adsorption behavior of the resulting samples towards murexide dye in aqueous solutions was investigated, along with their antibacterial efficacy against both gram-positive (Staphylococcus aureus and Bacillus) and gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacteria. The adsorption study revealed that the nanocomposite containing 9 wt% Fe3O4 NPs achieved the highest removal efficiency for MX (approximately 87.41 %) and exhibited a maximum adsorption capacity of approximately 100.7 mg/g, surpassing many adsorbents reported in the literature. Optimization experiments identified the optimal conditions as 40 mg of adsorbent dose, 40 mL of MX solution (125 mg/L), pH 5, and 160 min contact time. The adsorption kinetics followed pseudo-second-order kinetics, and the adsorption isotherm fitted well with the Freundlich model. Moreover, the nanocomposite was efficiently recovered from the solution using an external magnet, highlighting its potential for practical applications. Antibacterial testing revealed significant antibacterial effectiveness of PVTO and its nanocomposites against both gram-positive and gram-negative bacteria, with a minimum inhibition zone of 20 mm.