Ulva species are natural sources of nutrients and bioactive compounds; however, their chemical composition varies among species and with processing methods. In this study, the chemical profile and antioxidant activity of Ulva uncialis from Chile were determined, including both unprocessed and residual biomass. Additionally, polysaccharides from U. uncialis were chemically characterized, and their cytotoxicity, antioxidant, and immunostimulatory activities were evaluated in vitro after free-radical depolymerization. The results indicated that U. uncialis is rich in Ca, Mg, and Fe, and proteins remain in the biomass following ulvan extraction. Levels of Pb, Cd, Hg, and total As were below the limits established by the European Commission for Food Supplements. Both the ethanolic extract and ulvan exhibited radical-scavenging activity, and depolymerization improved the capacity of ulvan to stimulate cytokine production in RAW 264.7 cells. These findings suggest that U. uncialis is a promising source of nutritional and bioactive compounds.