Abstract: Monostroma is one of the economically important green seaweed worldwide, with its cultivation accounting for over 90% of the total cultivation volume of green seaweeds. The cell wall of Monostroma sp. contains abundant structurally unique sulfated polysaccharide, known as rhamnan sulfate. Rhamnan sulfate has not yet received extensive attention from researchers. Only limited studies reported its promising antiviral, anti-inflammatory, immunomodulatory, and anticoagulant activities. In this study, commercially farmed M. nitidum was used as the raw material, and rhamnan sulfate was isolated and purified via enzyme-assisted extraction and column chromatography. Its chemical structure was characterized by HPLC, FTIR, methylation analysis, and mass spectrometry. Its rheological properties were investigated combined with steady flow behavior and dynamic viscoelasticity analysis, and its lipid-binding capacity was evaluated via in vitro experiments. Results showed that rhamnan sulfate was a highly sulfated polysaccharide with a sulfate content of 28.47%, and its monosaccharide composition was dominated by rhamnose (71.34 mol%), followed by glucuronic acid (11.20 mol%), glucose (9.79 mol%), and small amounts of xylose and galactose. Comparative analysis of polysaccharide methylation before and after desulfation demonstrated that the backbone of rhamnan sulfate was mainly composed of →2)-Rhap-(1→ and →3)-Rhap-(1→ residues, with sulfate groups predominantly substituted at the C-4 of →2)-Rhap-(1→ residues. Mass spectrometry analysis of the hydrolyzed oligosaccharides of rhamnan sulfate indicated that some glucuronic acid residues were also substituted with sulfate groups. Steady flow behavior demonstrated that rhamnan sulfate solution exhibited shear-thinning behavior, with CaCl₂ addition inducing a remarkable viscosity enhancement. The polysaccharide maintained stable viscosity under acidic conditions and reached the maximum viscosity at pH 4, which is a superior advantage for acidic food applications. Dynamic viscoelastic analysis suggested rhamnan sulfate behaved as a weak gel. Additionally, rhamnan sulfate possessed excellent lipid binding capacities. The binding capacities of rhamnan sulfate toward triacylglycerol and cholesterol exhibited a decreasing trend with increasing polysaccharide concentration, with maximum binding capacities of 13.07 g/g and 117.47 mg/g, respectively. For sodium cholate, sodium taurocholate, and sodium glycylcholate, the maximum binding capacities were 52.52, 71.14, and 77.81 μmol/g, respectively. This study demonstrates that rhamnan sulfate from Monostroma possesses Ca²⁺-induced viscosity enhancement, stable viscosity in acidic environments, and outstanding lipid-binding capacity. These findings can provide theoretical support for the application of rhamnan sulfate in hypolipidemic functional foods and food additives, and promote the intensive processing and comprehensive utilization of Monostroma resources. Further studies should investigate the effects of rhamnan sulfate addition individually or in combination on food quality characteristics, providing more comprehensive evidence for its practical applications in food industry.