In Brittany, invasive algae represent a real problem, unbalancing local ecosystems and threatening the economy and tourism, with massive stranding’s, especially in summer. The valorization of invasive algae in green processes and materials such as ecological building materials could represent a real opportunity to try to control the spread of these species. Raw earth is a locally sourced building material, non-toxic and sustainable. In this work, we propose then to compare the effect of four invasive Brittany seaweeds, exotic or indigenous, on the fresh state and hardened properties of raw earth material. In particular, we investigated the performance of Polyopes lancifolius, Solieria chordalis, Ulva sp. and Sargassum muticum on the compressive strength, water resistance and extrudability of the raw earth material. Several forms of seaweed extracts were tested, raw seaweeds dried and ground, freeze-dried hot water extracts and extracted polysaccharides. Our results showed that the incorporation of S. chordalis and S. muticum, in the freeze-dried form or polysaccharides, allows a threefold increase in the compressive strength and a better resistance to water.