In this study, Se-doped FeO with antibacterial properties was synthesized using by a coprecipitation method. The chemistry and morphology of the Se doped FeO nanocomposite were characterized by energy-dispersive X-ray spectroscopy, field-emission scanning electron microscopy, X-ray diffraction, vibrating sample magnetometry, and Brunauer-Emmett-Teller spectroscopy. The antibacterial activity of the FeO/Se nanocomposite was examined against G (Gram-positive) and G (Gram-negative) bacteria, in the order Staphylococcus aureus, Staphylococcus saprophyticus, Pseudomonas aeruginosa, Klebsiella pneumonia, and Escherichia coli, which are the most harmful and dangerous bacteria. FeO/Se, as a heterogeneous catalyst, was successfully applied to the synthesis of pyrazolopyridine and its derivatives via a one-pot four-component reaction of ethyl acetoacetate, hydrazine hydrate, ammonium acetate, and various aromatic aldehydes. FeO/Se was easily separated from the bacteria-containing solution using a magnet. Its admissible magnetic properties, crystalline structure, antibacterial activity, mild reaction conditions, and green synthesis are specific features that have led to the recommendation of the use of FeO/Se in the water treatment field and medical applications. Direct Se doping of FeO was successfully realized without additional complicated procedures.