The development of a mixed beverage combining açaí pulp and whey unites the antioxidant potential of açaí with the nutritional and functional properties of whey, offering a promising option for health-oriented food products. This study evaluated the effects of thermal processing on the antioxidant properties, physicochemical parameters, and sensory acceptance of an açaí (Euterpe oleracea Mart.) pulp–whey-based beverage (APWBB). Samples were heated at 75 °C, 80 °C, and 90 °C for 15, 60, and 300 s, resulting in nine treatments (T1–T9). Thermal processing had little influence on the beverage’s physicochemical parameters; however, total phenolic content and total anthocyanins progressively decreased with increasing temperature and heating time. Losses reached 44.30 to 16.30 mg GAE/100 mL (phenolics) and 10.03 to 6.30 mg/100 mL (anthocyanins) between the control and the most intense treatment, showing a linear reduction pattern. These decreases were strongly correlated with reductions in antioxidant capacity (FRAP, DPPH, and ABTS), demonstrating the sensitivity of bioactive compounds to heat. Sensory evaluation revealed no significant differences among treatments, and the beverage showed high acceptance (71–80%) and positive purchase intention, highlighting its stability and market potential. Overall, the combination of açaí and whey represents a promising matrix for developing functional beverages, provided that suitable thermal parameters are adopted to minimize bioactive compound degradation.