Abstract: Background and Objective: Fucoidan is a sulfated polysaccharide with diverse biological activity including immune-potentiating activity, but such activity is little evaluated in fish. This research aimed to evaluate the non-specific immune-stimulating activity of fucoidan from a tropical brown alga, Padina boergesenii orally administered in catfish (Clarias sp.). Materials and Methods: A brown alga of P. boergesenii used was collected from Jepara intertidal coastal zone, Central Java, Indonesia and the species was identified base on morphological characteristics. Fucoidan was isolated by acidic methods. CaCl2 was added to separate fucoidan from alginate and then the fucoidan was precipitated by ethanol. Fucoidan was characterized by Fourier Transformed Infra Red (FT-IR) spectrometry and chemical analyses. Catfish was orally administered with fucoidan at 2,000, 4,000 and 6,000 mg kg–1 feed. Non-specific immune parameters of phagocytic activity, phagocytic index, superoxide anion (SOA) activity, superoxide dismutase (SOD), serum antibacterial activity, respiration burst and serum bacterial agglutination were evaluated. Results: Identification based on morphological characteristics of the brown alga used in this research was Padina boergesenii Allender and Kraft. FTIR spectrum of fucoidan extracted from P. boergesenii was identical to the bands demonstrated by FTIR spectrum of fucoidan standard from Fucus vesiculosus. Chemical analysis showed that fucoidan from P. boergesenii composed by sulfate 22.4%, uronic acid 7.5% and sugar content 40.18%. Dietary fucoidan increased SOA activity, serum antibacterial activity, serum agglutination and phagocytic activity (p<0.05). However, SOD activity and phagocytic index was not affected by supplemented fucoidan. Conclusion: This fucoidan significantly enhanced innate immunity of catfish at 4,000-6,000 mg kg–1 feed implying the potential of fucoidan to be used for feed supplement in stimulating non-specific immune response in fish culture.