In male anatomy, the lacuna magna (also called Guérin's sinus) is the largest of several recesses in the roof of the navicular fossa of the male urethra.

The lacuna magna is a large recess in the roof of the navicular fossa of the male urethra.

[1] The embryological origin of the lacuna magna is contested.

[2] However, recent evidence suggests it and the navicular fossa of the male urethra derive from infiltrating endodermal cells of the urethral plate.

[2] In young males, the presence of the lacuna magna is associated with painful urination (dysuria), bloody urine (hematuria), and bloody spotting of underwear.