Olecranon fossa is a depression situated directly superior to trochlea, at the posterior part of the distal end of the humerus. The olecranon process of ulna fits into olecranon fossa during full extension of elbow. Olecranon fossa is the largest of all three fossae in the region.
The capsule of elbow joint is connected above the coronoid and radial fossae anteriorly and above the olecranon fossa posteriorly. In some cases a bony bar between the coronoid and olecranon fossae is perforated and forms supratrochlear foramen.