This study examined the biochemical composition and bioactivity of three macroalgae (seaweed) species: Palmaria palmata, Ascophyllum nodosum, and Chondrus crispus. Nuclear Magnetic Resonance spectroscopy identified 18 metabolites, including amino and organic acids with significant interspecies variation. Inductively Coupled Plasma analysis showed high calcium (up to 49,340 mg/kg in C. crispus) and iron (728.9 mg/kg in C. crispus), with trace arsenic and lead present. High-Performance Liquid Chromatography detected carotenoids lutein (0.0105 mg/mL) and α-carotene (0.0010 mg/mL) in P. palmata. Gas Chromatography-Mass Spectrometry revealed A. nodosum rich in oleic acid (57.3 %), P. palmata in palmitic acid (82.8 %), and C. crispus in myristic acid (48.7 %). Antioxidant assays showed C. crispus (IC₅₀ 5.33 mg/mL, DPPH) and A. nodosum (IC₅₀ 0.25 mg/mL, ABTS) had highest activity. Significant antibacterial inhibition zones (up to 4.67 mm) and 66.6 % biofilm reduction by C. crispus were observed. These macroalgae represent promising natural bioactive sources for food and health.