The human penis is an external male intromittent organ that additionally serves as the urinal duct. The main parts are the root (radix); the body (corpus); and the epithelium of the penis including the shaft skin and the foreskin (prepuce) covering the glans penis. The body of the penis is made up of three columns of tissue: two corpora cavernosa on the dorsal side and corpus spongiosum between them on the ventral side. The human male urethra passes through the prostate gland, where it is joined by the ejaculatory duct, and then through the penis. The urethra traverses the corpus spongiosum, and its opening, the meatus (/miːˈeɪtəs/), lies on the tip of the glans penis. It is a passage both for urination and ejaculation of semen (see male reproductive system.)

Most of the penis develops from the same embryonic tissue as does the clitoris in females. The skin around the penis and the urethra come from the same embryonic tissue from which develops the labia minora in females.[1][2] An erection is the stiffening and rising of the penis, which occurs during sexual arousal. Erections can also occur in non-sexual situations; spontaneous non-sexual erections frequently occur during adolescence and during sleep. In its flaccid (unerect) state, the shaft of the penis has the feel of a dense sponge encased in very smooth eyelid-type skin. The glans of the penis, in uncircumcised males, is covered by the foreskin. In its fully erect (hard) state, the shaft of the penis is rigid, with the skin tightly stretched. The glans of the erect penis has the feel of a raw mushroom. An erect penis may be straight or curved and may point at an upward angle, a downward angle, or straight ahead. As of 2015, the best research on penis size concluded that the average erect human penis is 13.12 cm (5.17 inches) long and has a circumference of 11.66 cm (4.59 inches).[3][4] Neither age nor size of the flaccid penis accurately predicts erectile length.

The most common form of alterations of the penis are circumcision and piercings. Circumcision is the removal of part or all of the foreskin for various cultural, religious, and, more rarely, medical reasons, and there is controversy surrounding the practice.

Efforts by scientists to partially or fully regenerate the structures of the human penis are currently underway. Patients who can benefit most from this field are those who have congenital defects, cancer, injuries that have excised parts or all of their penis, and men wishing to reverse forms of involuntary genital modification.