The pyramid-shaped maxillary sinus (or antrum of Highmore) is the largest of the paranasal sinuses, located in the maxilla. It drains into the middle meatus of the nose through the semilunar hiatus. It is located to the side of the nasal cavity, and below the orbit.

It is the largest air sinus in the body. It has a mean volume of about 10 ml. It is situated within the body of the maxilla, but may extend into its zygomatic and alveolar processes when large. It is pyramid-shaped, with the apex at the maxillary zygomatic process, and the base represented by the lateral nasal wall.

It has three recesses: an alveolar recess pointed inferiorly, bounded by the alveolar process of the maxilla; a zygomatic recess pointed laterally, bounded by the zygomatic bone; and an infraorbital recess pointed superiorly, bounded by the inferior orbital surface of the maxilla. The medial wall is composed primarily of cartilage.

The nasal wall of the maxillary sinus, or base, presents, in the disarticulated bone, a large, irregular aperture that communicates with the nasal cavity. In the articulated skull this aperture is much reduced in size by the following bones:

The sinus communicates through an opening into the semilunar hiatus on the lateral nasal wall.

The medial wall is composed primarily of cartilage.

On the posterior wall are the alveolar canals, transmitting the posterior superior alveolar vessels and nerves to the molar teeth.

The floor is formed by the alveolar process, and, if the sinus is of an average size, is on a level with the floor of the nose; if the sinus is large it reaches below this level. Projecting into the floor of the antrum are several conical processes, corresponding to the roots of the first and second maxillary molar teeth; in some cases the floor can be perforated by the apices of the teeth.