The structure indicated is the sacrospinous ligament.
The sacrospinous ligament attaches at one end to the ischial spine, and at the other end to the lateral aspect of the sacrum and coccyx, lying deep to the sacrotuberous ligament. The sacrotuberous ligament runs from the sacrum to the ischial tuberosity.
The sacrotuberous ligament converts the greater sciatic notch into the greater sciatic foramen. Likewise, it converts the lesser sciatic notch into the lesser sciatic foramen.
Learn more about the bones of the pelvis in this tutorial.