Abstract        Plant pathogenic fungi, in particular are the source of several plant diseases,often regarded as one of the primary causes of decreased food supply around the world. The spread of such pathogenic fungi results in considerable crop losses degrades fruits and vegetables, reduces food availability, and costs billions of dollars annually. Although fungicides made of synthetic chemicals are commonly used for managing fungal diseases, they are harmful for the environment. Marine algae present an environmentally friendly option in the search for various kinds of compounds with industrial uses. They are one of the richest marine sources with a variety of bioactive compounds. Two green algae (Ulva flexusoa and Enteromorpha intestinalis) and one red algae (Griffithsia teges) are explored in the current work. The three seaweeds' methanolic extracts were evaluated in vitro for their biological activity against Penicillium expansum and Fusarium oxysporum mycelial growth. The antifungal activity of each of the extracts employed in this study was evident. The resulting inhibition zone in mycelial growth of Fusarium oxysporum as affected by  Enteromorpha intestinalis methanolic extractwas 37 mm, whereas that for Penicillium expansum was 32.6 mm. Gallic acid in the Enteromorpha intestinalis extract was 0.22 mg/ml, Chlorogenic acid was 0.41 mg/ml, Daizein was 0.16 mg/ml, Querectin was 0.64 mg/ml, Cinnamic acid was 0.05 mg/ml, and Hesperetin was 0.14 mg/ml, according to the results of the phenolic analysis carried out by HPLC-LC1620A.Also Ulva flexusoacontain high amount of Querectin 0.26 mg/mland Griffithsia teges contain high amount of  Gallic acid 0.81 mg/ml .These results suggest that the seaweeds employed in the study are a preferred and natural source of bioactive chemicals. Such natural products may have applications in the management of fungal diseases in organic farming and other sustainable agricultural practices.