Low-fat or reduced-fat food products have received significant attention in recent years. In the present study, the physicochemical, nutritional, and functional properties of reduced fat mayonnaise were evaluated. This mayonnaise was formulated by partially substituting oil with modified potato peel starch and isolated soy protein. Oil substitution levels ranged from 5% to 15% using the modified starch gel. Reduced-fat mayonnaise samples were compared to full-fat mayonnaise (control) in terms of physicochemical properties and sensory attributes. The amylose content of potato peel starch before and after modification was approximately 24.01% and 33.01%, respectively. The water-holding capacity, oil-holding capacity, protein yield, and protein content of soy protein isolate were found to be 1.12 ml/g, 1.02 ml/g, 30.49%, and 86.85%, respectively values that were higher than those of the modified potato peel starch. At the beginning of cold storage (4°C), the pH values of the mayonnaise samples ranged from 3.62 to 3.94, increasing to 4.49–4.99 after 30 days. High emulsion stability was maintained with up to 10% oil replacement. The full-fat control sample showed the highest L* (brightness) and b* (yellowness) values, followed by the sample containing 5% modified starch gel throughout the storage period. The reduced-fat mayonnaise with 15% modified starch gel had a total calorie content 21.48% lower than the full-fat mayonnaise. Compared to the control, mayonnaise samples with 5% and 10% modified potato peel starch substitution demonstrated superior sensory characteristics. The co-addition of isolated soy protein and modified potato peel starch did not adversely affect the physicochemical or sensory properties of the reduced-fat mayonnaise, except for a reduction in viscosity due to starch gel addition, even after one month of storage. These results suggest that using modified potato peel starch as a value-added component of agro-industrial waste, in combination with soy protein isolate, offers a promising technological approach for developing reduced-fat mayonnaise with enhanced health benefits.