Cassava‐soy composite is extruded with grape pomace at 0, 10, and 20% addition levels. Some nutritional, functional, rheological properties, total phenolic content, and antioxidant activity of the extruded products are analyzed. Kinetics of starch digestibility show that extrusion cooking leads to a significant (p < 0.05) increase in the rate of starch digested but the addition of grape pomace lowers the rate of starch digestibility and estimated glycemic index. The water absorption capacity of composites decreases while the solubility index increases with increasing addition of grape pomace. FTIR spectra show an increase in β‐sheet formation as the addition of grape pomace increases. The composite with 20% grape pomace has the lowest viscosity and all composites exhibit a shear thinning behavior. Extrusion cooking probably leads to depolymerization of dietary fiber and redistribution of insoluble dietary fiber to soluble dietary fiber ratio. The total phenolic content and antioxidant activity of the cassava‐soy composite are significantly (p < 0.05) increased with the addition of grape pomace. The results indicate that grape pomace can be added to starch‐rich foods with extrusion cooking to produce instant products with potential health promoting properties.