This study aims to analyze the chemical composition of aqueous extracts from different parts of Cleome arabica and evaluate their antifungal activity against the phytopathogenic fungi Alternaria alternata and Fusarium proliferatum, isolated from post-harvest potato tubers in the El Oued region. The aqueous extracts were obtained through maceration under agitation for 24 hours. Phytochemical tests conducted on the aqueous extracts revealed the presence of various chemical compounds, including tannins, flavonoids, saponins, alkaloids, and reducing compounds. HPLC analysis revealed that the aqueous extract of Cleome arabica leaves contains acetylsalicylic acid, ascorbic acid, gallic acid, and pyrogallol. In contrast, the stem extract is characterized by the presence of vitamin D3 and ascorbic acid, while the root extract contains gallic acid and vitamin B1. The antifungal activity of the aqueous extracts was evaluated using the direct contact method on the mycelial growth of the tested strains.The results highlighted a strong antifungal potential against these strains. The aqueous extract of Cleome arabica leaves exhibited 100% antifungal inhibition against Fusarium proliferatum and Alternaria alternata at concentrations of 1%, 10%, and 15%. The aqueous extract from the stem of Cleome arabica showed total antifungal activity (100%) against both tested fungi at concentrations of 10% and 15%. Regarding the root extract, concentrations of 10% and 15% resulted in complete (100%) inhibition of Alternaria alternata and Fusarium proliferatum. These findings highlight the potential of Cleome arabica leaves, stems, and roots as a natural alternative to synthetic chemical fungicides.