The sigmoid sinus is a dural venous sinus that lies deep within the human head, and just below the brain. A dural sinus is a channel that lies between the dura mater, the outermost tissue covering the brain. The sigmoid sinus is actually a pair of two sinuses (right and left) that enable veins to spread from the middle of the head downwards. This dual structure is considered an extension of the transverse sinus, which lies in the hind portion of the brain.

The sigmoid sinus starts just below the temporal bone on the sides of the head and follows a complex course to the jugular foramen, a hole in the bone at the base of the skull. At this point in its course the sinus meets with the internal jugular vein.

As the tentorium (the dura mater that separates the cerebellum, located at the back of the head, from the rest of the brain) reaches its end, the sigmoid connects with the petrosal sinus, which is above it in terms of structure. The sigmoid sinus moves in a downward manner through an S-shaped groove. This groove lies behind the middle of the mastoid, an area of bone behind the ear. It is connected through the mastoid and the condylar messenger veins with the pericranial nerves.

The sigmoid and transverse sinus both vary in size.