The emulsifying properties of the ethanol‐precipitated mucilage from cellulase‐hydrolyzed Monostroma nitidum were investigated. Both emulsifying activity and emulsion stability of the mucilage increased from 51.5 and 48.0% to 69.0 and 62.0%, respectively, and oil droplets did not coalesce when the mucilage concentration was at l%(w/v). Gel permeation chromatography showed that the carbohydrate moieties of the mucilage exhibited an absence of low molecular weight fractions and a broader molecular weight range for the high molecular weight fraction than that of the cellulase hydrolysate. Also, there was a greater amount of high molecular weight protein‐containing fractions in the mucilage, while predominantly low molecular weight protein fractions were in the hydrolysate. Increasing the mucilage concentration of the solution(0–20 mg/mL) led to a reduction of surface tension to 54.9 dyne/cm, and increased the emulsifying activity and emulsion stability to 76.9 and 70.0%, respectively. The oil did not separate from the emulsion when the mucilage concentration was higher than 0.5%(w/v). When 0.2% NaCl was added to the emulsion containing 1% mucilage, the apparent viscosity decreased from 37.1 to 7.6 Pa.s (x 10‐3). Also, the emulsifying activity and emulsion stability decreased from 69.0 and 62.0% to 50.0 and 48.0%, respectively.