Air sac is an anatomical term with several meanings:

• Pulmonary alveolus, informally known as an air sac, one of innumerable spherical outcroppings of the respiratory bronchioles in the mammalian lung, the secondary sites of gas exchange with the blood
• an anatomical structure continuous with the trachea found in some insects which can be used to actively pump air throughout the body
• an anatomical structure unique to the dinosaur and bird respiratory systems that allows unidirectional flow of air into the lungs and through the body