Green seaweeds are rich sources of sulfated polysaccharides (SPs) with potential biomedical and nutraceutical applications. The aim of this work was to evaluate the immunostimulatory activity of SPs from the seaweed, Caulerpa cupressoides var. flabellata on murine RAW 264.7 macrophages. SPs were evaluated for their ability to modify cell viability and to stimulate the production of inflammatory mediators, such as nitric oxide (NO), intracellular reactive oxygen species (ROS), and cytokines. Additionally, their effect on inducible nitric oxide synthase (iNOS) and cyclooxygenase 2 (COX-2) gene expression was investigated. The results showed that SPs were not cytotoxic and were able to increase in the production of NO, ROS and the cytokines, tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6). It was also observed that treatment with SPs increased iNOS and COX-2 gene expression. Together, these results indicate that C. cupressoides var. flabellata SPs have strong immunostimulatory activity, with potential biomedical applications.