A square planar platinum(II) complex [PtL₂] was prepared from 1-phenyl-3-(pyridin-2-yl)thiourea (HL) and potassium tetrachloroplatinate(II), and X-ray crystallography revealed its orthorhombic packing. Rhizopus stolonifer is a phytopathogen that infects many types of fruits and vegetables. Against Rhizopus stolonifer, this complex exerted a low IC₅₀ of 36.8 µg/mL, indicating strong antifungal potency (nystatin provided a value of 68.4 µg/mL). The complex demonstrated its ability to defend against Rhizopus stolonifer by providing enhanced fungal dry mass, sugar consumption, soluble proteins, and total antioxidants compared to the standard; specifically, [PtL₂] and nystatin at 100 µg/mL resulted in dry masses of 0.55 ± 0.05 and 1.42 ± 0.07 g/L, sugar consumptions of 25.4 ± 0.37% and 49.7 ± 0.04%, and soluble proteins of 9.34 ± 0.03 and 7.35 ± 0.04 mg/g fungal weight. The total antioxidants provided by nystatin and [PtL₂] at 100 µg/mL are 6.95 ± 0.019 and 9.49 ± 0.047 mg/g protein, respectively, while [PtL₂] at 80 µg/mL offered total antioxidants of 10.05 ± 0.039 mg/g protein.