The effect of sweet whey and octenyl succinic anhydride (OSA)-modified cassava starch on the quality and rheological properties of fermented dairy beverages was evaluated. Sweet whey (45-65%) and OSA-modified cassava starch (0.8-1.2%) were added to determine an optimal fermented dairy beverage with the highest viscosity and the lowest syneresis possible. The optimal fermented dairy beverage corresponded to the addition of 40.9% sweet whey and 1.13% OSA-modified cassava starch with respect to the milk and sweet whey mixture. Moreover, the rheological and quality properties of the optimal fermented dairy beverage were compared to a commercial beverage (control) during 22 days of storage. No significant differences were found in soluble solids, acidity, pH and consistency index during the time evaluated, while the syneresis of both products showed an increase during storage. OSA-modified cassava starch can be used as a stabiliser in sweet whey fermented dairy beverages because it helps improve its quality properties.