This study is based on the synthesis of 4-phenylazo-1-naphthol (C₁₆H₁₂N₂O) as an azo coupled dye through coupling reaction of phenyl diazonium salt and α- naphthol in ice-cold chilled water. Azo coupling involved an electrophilic substitution reaction of phenyl diazonium cation with α- naphtholate ion, the coupling partner. The 4-phenylazo-1-naphthol was characterized through fourier transform infrared spectroscopy (FTIR). Antifungal activity of 4-phenyl-azo-1-naphthol was checked against Fusarium oxysporum f. sp. lycopersici (FOL), a soil-borne fungal pathogen causing wilt disease in tomato. Eight concentrations ranging from 0.78 to 100 mg mL^-1 were tested against the fungus. None of the concentration suppressed the fungal growth. Instead, all the concentrations variably enhanced the fungal biomass over control by 8–28%. This study concludes that the synthesized compound did not possess antifungal potential against FOL.