Acid whey (AW) has become a significant challenge for the dairy industry because of the large quantity produced and the difficulty in further processing and economical disposal. This study investigated the feasibility of spray drying AW using millet flour as encapsulating wall material and examined the effect of different millet concentrations on the physio-functional properties of the newly developed acid whey millet (AWM) powders. The results showed that the millet flours had a neutralizing effect on AW, which aided the spray drying of AW by increasing the glass transition temperatures, thereby reducing the stickiness of the powders. Millet flours added to AW improved the yield, color, and reconstitution properties of AWM powders. AWM powders had a high concentration of phenolic compounds and exhibited high antioxidant activity. The results from this study indicate that millet flour can be used as a suitable wall material for spray drying AW and developing powders with potential applications as functional food ingredients.