Hybrid nanoparticles (NPs) have received much interest over the past decades because they have the potential to overcome the limits of single-component particles. This study proposes a hybrid magnetic magnesium oxide (m-MgO) NPs to combat the plant pathogenic fungus, Fusarium oxysporum (F. oxysporum). The m-MgO NPs were synthesized via ultrasonic mediated sol-gel method. UV-visible spectrometry confirms the successful formation of m-MgO NPs. In addition, the magnetic activity of m-MgO NPs was illustrated through a preliminary magnetic activity study. A disc diffusion assay was carried out to determine the effectiveness of m-MgO NPs to inhibit the growth of F. oxysporum. The results showed that the zone of inhibition was 7.58 ± 0.30 mm at 10 mg/mL, suggesting that the synthesized m-MgO NPs are an effective fungicide to inhibit the growth of F. oxysporum.