<p>The aim of this study was to enhance the nutritional value and antioxidant bioaccessibility of gluten-free biscuits by incorporating increasing amounts of chia seeds. The physical, chemical, and sensory properties, as well as the in vitro bioaccessibility of total phenolics and antioxidant activity, were evaluated in gluten-free sweet biscuits developed by substituting red rice flour and tapioca starch with chia seeds (0–20%). The changes in sensory, textural, and some chemical properties of the biscuits during six months of storage were also monitored. As the chia seed content increased, the moisture, ash, protein, fat, and total dietary fiber contents of the biscuits rose, while the carbohydrate content decreased. Biscuits with 20% chia substitution exhibited the highest nutritional value, particularly in dietary fiber content (6.16%), while those with 10% chia substitution maintained overall acceptability scores comparable to the control biscuits throughout storage. The total phenolic content and antioxidant activities (DPPH and FRAP) of all chia-substituted biscuits were significantly higher than those of the control biscuits. Additionally, incorporating chia seeds improved the bioaccessibility of phenolic compounds and antioxidants, with biscuits containing 10% chia substitution showing the highest bioaccessibility for both phenolics and FRAP.</p>