Fucoidan is a sulfated polysaccharide isolated from edible marine resources that may be used as a functional ingredient in foods because of its characteristic physicochemical properties and biological activities. In this study, the impact of a fucoidan extracted from sea cucumber on the properties of protein-coated oil-in-water emulsions was studied. The molecular characteristics of fucoidan in aqueous solution were characterized by electrophoresis and size-exclusion chromatography. The influence of fucoidan addition on the electrical charge, creaming stability, size, and microstructure of the particles in caseinate-stabilized emulsions was then measured. Fucoidan adsorbed to the surfaces of caseinate-coated droplets from pH 2 to 6, due to electrostatic attraction between anionic groups on the polysaccharide chains and positive groups on the adsorbed proteins. Like other anionic polysaccharides, the electrostatic deposition of fucoidan improved the pH stability of protein-stabilized emulsions near their isoelectric point. The phase behavior of droplet-polysaccharide mixtures (i.e. bridging flocculation, stabilization, and depletion flocculation) depended on lipid droplet and fucoidan concentration. Transmission electron microscopy suggested that fucoidan formed a relatively thin layer (a few nanometers) around the protein-coated oil droplets, suggesting that it laid fairly flat against the droplet surfaces. This study indicates that sea cucumber fucoidan may be a suitable functional ingredient for stabilizing protein-coated emulsion droplets against flocculation through an electrostatic deposition mechanism.