The aim of this study was to prepare the refined oil derived from Sardinella longiceps by a step-wise procedure of degumming, bleaching, and deodorization. The refined sardine oil showed greater susceptibility toward oxidation than the crude oil, but the addition of ethyl acetate fractions of the red seaweeds Hypnea musciformis, Kappaphycus alvarezii, and Jania rubens with great antioxidant potential significantly increased the oxidative stability of the refined oil. The refined oil supplementedwith the ethyl acetate fraction of these seaweeds indicated more noteworthy oxidative stability indices (≥ 0. 51 h) than α-tocopherol. The induction time was greater for refined oil added with H. musciformis (1.26 h) than butylated hydroxytoluene (1.04 h) and α-tocopherol (0.4 h). No traces of aromatic (δ 6.7–7.5 ppm), hydroperoxides, and aldehydes (δ 9–10 ppm) in the 1H-NMR spectra of the refined oil supplemented with seaweed extracts at the baseline and after the accelerated storage study demonstrated that the seaweed extracts were able to prevent the formation of secondary oxidation products in the unsaturated system during storage. This study demonstrated the potential of seaweeds as natural alternatives to the synthetic antioxidants to prevent the rancidity of the refined fish for use in food and pharmaceutical industries.