The screening of potential antioxidant activities of hydrophobic (ethyl acetate) and hydrophilic (polysaccharide) extracts from both mycelia and fermentation media of eleven algicolous fungi was performed using three antioxidant assays. Algicolous fungi were able to ferment low-cost nutrients composed of potato infusion, glucose, and natural seawater and produce hydrophobic and hydrophilic metabolites with antioxidant properties. Significant differences were observed in antioxidant activities of extracts from the various species evaluated using total antioxidant activity (TAA), hydroxyl radical scavenging activity (HRSA), and ferric reducing antioxidant power (FRAP), as well as their total sugar contents (TSC) of hydrophilic extracts and total phenolic contents (TPC) of hydrophobic extracts. TSC of endopolysaccharides was higher than exopolysaccharides and showed a significant correlation with both TAA and FRAP. Most of mycelial extracts were richer in phenolics than fermentation media, and TPC of hydrophobic extracts was significantly correlated with their FRAP. Mycelial extracts for most of algicolous fungi showed higher antioxidant activities than fermentation media. Both hydrophobic and hydrophilic extracts of algicolous fungi showed good antioxidant properties, especially as hydrogen-donating antioxidants, which could be considered for future applications in medicine, food production, or cosmetic industry.