Abstract text: Green algae inhabit aquatic environments across the planet and play a crucial role in sustaining the global ecosystem. Ancestors of some Charophyte lineages adapted to terrestrial conditions and eventually evolved into land plants. Extant green algae have inherited traits from their ancestors and evolved into their current morphological and chemical forms, as represented by their cell walls with distinct shapes and compositions. To gain a deeper understanding of the evolution of the plant cell wall and to bridge the gap between land plants and early-diverging green algae, we investigated Penium margaritaceum and isolate a previously unknown polysaccharide from the culture medium of Penium as well as from its cell wall. This novel polysaccharide features a plant RG-I-like structure, consisting of repeats of [-3-α-Fucp-(1,4)-α-GlcpA-] disaccharide that are highly xylosylated and acetylated. A survey of approximately 20 non-vascular plant species suggests that the novel polysaccharide is widely distributed across lower plants. The high degree of similarity between the repetitive structures of the xylosylated fucoglucuronan and [-2-α-L-Rhap-(1,4)-α-D-GalpA-] in RG-I, as well as the repetitive [-4-β-D-GlcpA-(1,4)-α-L-Rhap-3-sulfate-] structure of ulvan, raises the question of how cell walls, particularly RG-I-like structures, have evolved in plants from marine to freshwater environments and, ultimately, to land.