The objective of this study was to explore the use of doum fruit powder (Hyphaene thebaica L.) as a functional ingredient in gluten-free oat-based cookies. Doum fruit powder (DFP) was used to replace oat flour at 5, 10, 15 and 20% levels, and the resulting cookies were analyzed for nutritional, antioxidant, physical, color and sensory characteristics. DFP had higher dietary fiber, mineral, phenolic compound and antioxidant activity content than oat flour. Increasing the DFP level significantly increased the crude fiber (5.81–8.69%), total phenolic content (37.08–50.04 mg GAE/100 g), antioxidant activity (15.67–24.11%) and mineral content, and decreased the caloric value. Higher substitution levels also produced darker cookies with lower volume and higher density. Although sensory scores decreased with increasing DFP levels, all formulations remained acceptable. Correlation analysis and principal component analysis revealed strong associations between fiber, phenolics, antioxidant activity, and mineral content, while indicating a trade-off between nutritional enhancement and sensory quality. Cookies with 5–10% DFP provided the best compromise between the improved nutritional value and the acceptability by consumers. The results proved that DFP can be successfully employed to produce functional gluten-free cookies with improved health-promoting properties.