Traditionally, intensive thermal treatments are applied to process soy beverages, negatively impacting product quality by reducing solubility, and viscosity, while promoting syneresis. Modified cassava starches can prevent these effects. This study evaluates the influence of adding modified cassava starches on syneresis, apparent viscosity, particle size, microstructure, and sensory acceptance of a fermented soy beverage. A commercial culture of starter microorganisms and probiotics is used in the fermentation. Four beverages are produced: one with native cassava starch, one with octenyl succinic anhydride (OSA)‐modified cassava starch, one with crosslinked acetylated distarch adipate (ADA), and one with crosslinked‐substituted cassava starch (mixed); the starch concentration is 1% in each beverage. Samples are stored for 3 weeks at 4 °C. Modified cassava starches significantly alters syneresis compared to the native starch beverage, with no difference among the other treatments. Viscosity shows significant differences among treatments. During storage, the native starch beverage shows an increase in syneresis and a reduction in viscosity, whereas the OSA‐modified starch beverage has higher viscosity but presents significant variation by the end of the 3‐week storage period. The mixed starch beverage has intermediate characteristics with high viscosity and stability during storage. In the sensory evaluation, the consumers prefer the OSA‐modified starch beverage.