Sodium alginates (SA) were extracted and purified from apices resulting as waste of the ex-situ cultivation step for restoration of the brown seaweed Ericaria amentacea in the Ligurian Sea, employing hydrothermal-alkaline extraction. The structural characterization of SA was obtained by FTIR and 1H-NMR. The bioactive properties of this polysaccharide were also investigated, in terms of in vitro antioxidant activities and of anti-inflammatory properties in LPS-stimulated macrophages by assessment of inhibition of inflammatory cytokines and mediators. The FTIR analysis of SA exhibited O-H, C-H, and COO- stretching which are represented by bands at 3301 cm−1, 2926 cm−1, and 1597–1408 cm−1, respectively, compared to commercial alginate. The 1H NMR spectroscopy indicated that the extracted sodium alginates have a high content of β-D-mannuronic (M) than α-L-guluronic acid (G) with M/G ratio 2.33. SA exerted significant antioxidant activities with high DPPH and nitric oxide radical-scavenging potential, together with a moderate ferric reducing potential. Moreover, the data obtained in this study demonstrate a strong anti-inflammatory effect of sodium alginate polysaccharide. Indeed, SA were able to strongly inhibit LPS-induced inflammatory mediators, such as IL-1β, IL-6, IL-8, MCP-1, iNOS and TNF-α gene expression in human THP1-M0 macrophages as well as in Raw 264.7 murine macrophages. These results pave the way for the future use of E. amentacea metabolites, particularly if sustainably obtained from waste of restoration activities, in industrial and biomedical applications.