Fortification of aquaculture foodstuff with various algae may improve the resistance of certain fish or shrimp to diseases and, as a routine procedure, has become ever more popular and, seemingly, important. Herein, we isolated certain alkali-soluble polysaccharides from a Rhizoclonium riparium alga (RASP), polysaccharides that can be separated into two different groups on the basis of the polysaccharide's molecular weight. Using gas chromatography-mass spectometry analysis, we found that the major monosaccharides constituting the higher molecular-weight group of RASP were galactose (41.99%), glucose (34.53%), xylose (20.24%), and mannose (3.24%). Using a murine-derived macrophage cell line J774A.1, we found that polysaccharide constituents of the higher molecular-weight group of RASP were able to induce interleukin-1beta (IL-1) gene expression via protein kinase-mediated signal transduction pathways. In essence, we found that c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38), but not extracellular signal-regulated kinase (ERK), play an important role in the regulation of IL-1 gene expression in RASP-stimulated J774A.1 cells. To the best of our knowledge, this is the first occasion that polysaccharides from R. riparium have been demonstrated to exert immunomodulation properties by the induction of IL-1 within macrophages. Our current results provide support for the possible use of R. riparium as an additive to various food/foodstuff, to modulate the immune response of humans or certain animals.