Seaweeds contain huge amounts of bioactive compounds that can be used for medicinal, nutritional, and agricultural purposes. This study analysed the antioxidant and antimicrobial properties as well as the content of bioactive compounds in twelve seaweeds and highlighted their use against aluminium toxicity in Trigonella foenum-graecum (fenugreek) seedlings. Among the tested seaweeds, the brown species Eisenia bicyclis and Sargassum horneri had the highest total antioxidant activities. They could scavenge 74% and 77% of 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) free radicals, respectively. Thus, gas chromatography/mass spectrometry (GC/MS) analysis was performed for these two algal species, and reports showed some valuable compounds, such as phthalic acid, stigmasterol, and palmitic acid which are medically important for human health and stimulators for plant growth. The amounts of polyphenols (maximum 303.28 mg GAE/g DW) and flavonoids (maximum 45.44 mg CE/g DW) were higher in brown seaweeds than in green or red ones. Our results revealed that extracts of Eisenia bicyclis and Sargassum horneri exhibited antimicrobial effects against four microorganisms. Also, a bioassay was done using fenugreek seedlings subjected to aluminium stress and/or seaweed extracts. It was found that seedling growth was inhibited by Al treatment, while they enhanced when germinated in extracts of S. horneri and E. bicyclis, alleviating Al toxicity in the used concentrations. The results of our study shed light on the use of seaweed extracts as sources of bioactive compounds against aluminium stress in fenugreek seedlings and as natural antimicrobials in food preservation, pharmaceuticals, and other industries; their use is safe, low cost, and eco-friendly.