We investigated the phytochemical content and antioxidant properties of four brown seaweeds: Cystoseira myrica, Dictyota spiralis, Sargassum euryphyllum, and Turbinaria decurrens. The antidiabetic potential of the α-amylase and α-glucosidase starch hydrolyzing enzymes of different brown algal extracts was also evaluated. All of the seaweeds analyzed had antioxidant effects, as assessed using 2,2-diphenyl-1-picrylhydrazyl (DPPH), reducing power, hydrogen peroxide radical scavenging, and total antioxidant capacity analyses, in addition to antidiabetic efficacy. These effects were influenced by the species and the solvent used. Among the three solvents, water extract had a higher content of phenol, tannins, and flavonoids than the other solvents, methanol and hexane. All extracts showed antioxidant activities which had a significant relationship with their phytochemical content. Water extract had greater antioxidant activity than the other extracts in all assays except the DPPH assay. Water extract of T. decurrens had the highest inhibitory effect on α-amylase and α-glucosidase. The best algal extracts were analyzed using gas chromatography-mass spectrometry (GC–MS), which revealed the existence of different compounds with various chemical structures. These compounds are known to have antioxidant and antidiabetic activities. It can be concluded that the prominent bioactive compounds found in brown algae have the potential to treat a wide range of serious diseases. The potential activities of specific biocompounds may be valuable in the pharmaceuticals, cosmeceuticals, and functional foods industries.