A combination of biopolymers of both proteins and polysaccharides are commonly used to increase the stability in many colloidal food systems. This study examined the effect of fucoidan on the surface activity of bovine serum albumin (BSA) and the emulsifying properties of BSA-stabilized oil-in-water emulsions at different pH. BSA lowered the surface tension significantly from 71 to 55 mN at the air-water interface, but fucoidan did not. In the experiment in the BSA-fucoidan mixture, there was no synergistic effect between BSA and fucoidan, indicating that fucoidan was not surface-active. At pH 5.0, only the BSA-added emulsion was drastically destabilized due to protein aggregation and desolubilization. However, the addition of 0.05 wt% fucoidan reduced the creaming rate, recovered the emulsion phase stability, and reduced the droplet size significantly. Interestingly, at pH 3.0, which is below pI, either the BSA-added emulsion or BSA-fucoidan mixed emulsion remained stable, suggesting the contribution of strong repulsions with positive and negative charges into the emulsion. Consequently, although fucoidan is not surface-active, this anionic polysaccharide acts as an electrolyte that can form a highly negative environment, and contribute to stabilizing the emulsion under any pH conditions.