This study investigated the effects of yeast protein (YP) as a fat replacer on the rheological properties, emulsion stability, and microstructure of reduced-fat mayonnaise. Owing to its favorable structural and functional characteristics-including an ordered and compact molecular structure, high water and oil holding capacities, and excellent emulsifying stability-YP-1 demonstrated outstanding performance as both an emulsifier and stabilizer. At a 40 % replacement level, sensory evaluation indicated acceptable quality. YP-based fat replacement significantly enhanced emulsion stability, with all samples maintaining over 95 % stability without oil-water separation. As the level of YP increased, the mayonnaise exhibited larger droplet sizes and higher viscosity. Thixotropic testing showed that structural recovery improved with increasing YP content, reaching a maximum of 99.12 % at 40 % replacement. Additionally, microstructure illustrated that all mayonnaises existed as emulsion droplets and YP did not change the network structure. None of the samples showed excessive aggregation or flocculation.