Fungal phytopathogens such as Alternaria, Cercospora, Colletotrichum and Fusarium cause substantial yield and quality losses, while increasing resistance and environmental concerns limit the long-term use of synthetic fungicides. This study evaluated the in vitro antifungal activity of aqueous, methanolic and acetone leaf extracts of Cassia fistula L. against these four fungi and related the activity to their phytochemical profiles. Shade-dried, defatted leaf powder was extracted by Soxhlet using water, methanol and acetone, followed by qualitative phytochemical screening and HPTLC fingerprinting. Antifungal activity was assessed by the poisoned food technique on potato dextrose agar, recording colony diameter at 3 and 5 days after inoculation and calculating percentage inhibition over control; data were analysed by ANOVA and Tukey’s post hoc test. Tannins, flavonoids and alkaloids were detected in all extracts, whereas carbohydrates and proteins were predominantly present in the aqueous extract, and methanolic and acetone extracts showed stronger reactions for phenolic and flavonoid constituents. HPTLC profiles of methanolic and acetone extracts exhibited more numerous and intense bands than the aqueous extract, indicating a richer diversity of moderately polar compounds. All extracts significantly inhibited mycelial growth of the test fungi, but methanolic extract consistently produced the highest inhibition, followed by acetone, while the aqueous extract showed comparatively lower activity; inhibition increased with extract concentration and was generally higher at 5 than at 3 days after inoculation. Alternaria and Cercospora were relatively more sensitive, whereas Fusarium displayed lower susceptibility at equivalent treatment levels. The results suggest that C. fistula leaves, particularly methanolic and acetone extracts enriched in phenolic and flavonoid constituents, represent promising sources of botanical antifungal agents for integration into eco-friendly management of fungal plant diseases.