Over the past two decades, intensive work has been conducted on antioxidants as they have great potential to prevent diseases and complications associated with age. Seaweeds are rich natural sources of bioactive metabolites with antioxidant and antimicrobial characteristics. In the present study, bioactive metabolites were extracted from the seaweed Gelidium pusillum by maceration in aqueous and ethanolic medium. The dried weed powder was analyzed by GC-MS and FTIR. The macerated products were assayed for antioxidant and antibacterial activity by in vitro and in vivo methods. Antibacterial activity against the marine pathogen Aeromonas caviae was observed. Giant freshwater prawns (Macrobrachium rosenbergii) were used as models for in vivo assays by inducing pathogenicity by intramuscular injection of pathogen. The present study revealed that G. pusillum is a rich source of antioxidant and antibacterial compounds. The data obtained from the present study substantiates the potential of G. pusillum as an ideal antioxidant and antibacterial in the aquaculture industry. The animals treated with the ethanolic extract of G. pusillum (35 μl of 1mg/ml extract) survived even after 3 weeks, compared to the untreated in which 100% mortality was observed within 120 h. Relative percentage survival (RPS) of 90% was observed in the group treated with the seaweed extract. GC-MS analysis revealed the presence of several compounds previously reported for antibacterial activity. In the present work, a total of seven compounds were identified by GC-MS analysis, out of which two compounds are reported to have antimicrobial activity. High sustained survival of the infected prawns post-administration of the extract revealed that this sea weed has potential in the biopharmaceutical, aquaculture, and food sectors. © 2021, The Author(s), under exclusive licence to Springer Nature Switzerland AG part of Springer Nature.