In anatomy, Underwood's septa (or maxillary sinus septa, singular septum)[1][2] are fin-shaped projections of bone that may exist in the maxillary sinus, first described in 1910 by Arthur S. Underwood, an anatomist at King's College in London.

[3] The presence of septa at or near the floor of the sinus are of interest to the dental clinician when proposing or performing sinus floor elevation procedures because of an increased likelihood of surgical complications, such as tearing of the Schneiderian membrane.

[4] The prevalence of Underwood's septa in relation to the floor of the maxillary sinus has been reported at nearly 32%.

Primary septa are those initially described by Underwood and that form as a result of the floor of the sinus sinking along with the roots of erupting teeth; these primary septa are thus generally found in the sinus corresponding to the space between teeth, as explained by Underwood.

Conversely, secondary septa form as a result of irregular pneumatization of the sinus following loss of maxillary posterior teeth.