The current study examined the biochemical composition of four seaweeds (Ulva fasciata, Ulva compressa, Corallina officinalis, and Corallina elongate) which were collected from Eastern Harbor located at Alexandria Mediterranean coast, Egypt. Total chlorophyll content was the maximum in Ulva compressa (2.7 mg g^–1 FW) and the minimum value was observed in Corallina elongate (0.90 mg g^–1 FW). In comparison, the maximum carotenoids were registered in Corallina officinalis (1.04 mg g^–1 FW) followed by Corallina elongate (0.86 mg g^–1 FW). The lowest ratio was recorded in Ulva fasciata (0.45 mg g^–1 FW). The results showed that the green seaweed (Ulva compressa) contained the highest amounts of phenols (12.7 mgGA/g dry wt.), flavonoid (9.42 mgCA/g dry wt.) and has the maximum percentage of DPPH radical scavenging capacity, Total antioxidant capacity (TAC) assay, and Total Reducing Capacity (TRC)  (80.45, 12.5 and 71.6 respectively). On the other hand, the red seaweed (Corallina elongate) contained the lowest amounts of phenols (5.9 mgGA/g dry wt.), flavonoid (8.29 mgCA/g dry wt.), and has the maximum percentage of DPPH, TAC, and TRC (70.3, 8.4, and 57.8 respectively). Due to seaweed's biochemical composition, these findings recommend being used as an antioxidant agent for food supplements, cosmetics, medicinal applications, and pharmaceutical industries.