Casein protein powder is generally considered gluten-free. Casein is a type of protein that is found in milk and is not derived from gluten-containing grains like wheat, barley, or rye. However, it is important to check the labeling and manufacturing processes of the casein protein powder to ensure that it has not been cross-contaminated with gluten during production. If you have celiac disease or a gluten intolerance, it is always best to consult with your doctor or a dietitian before using any new protein powder.