The lion is the king of the beasts, and as such is usually the first beast described in the bestiaries. The lion chapter is generally one of the longest and most complex.

The lion has these natures: When a lion walking in the mountains sees that it is being hunted, it erases its tracks with its tail; it always sleeps with its eyes open; a lion only kills out of great hunger; it will not attack a prostrate man, but will attack a man who looks at it; it allows captive men to depart; it is not easily angered.

Unlike most animals, lions mate face to face. The lioness give birth to five cubs the first year, then four the next, and three the next, until after the birth of a single cub in the fifth year, she becomes barren and has no more cubs. Her cubs are born dead and are brought to life on the third day when the mother breathes in their faces or the father roars over them.

There are two kinds of lion: one is timid, has a short body and curly hair; the other has straight hair and a long body and is fierce. A lion's strength is seen in its chest, its firmness in its head, and its courage in its forehead and tail.

Lions are frightened of the sight of hunters with spears, so they look at the ground when surrounded. They also fear the sound of creaking cart wheels, fire, and the sight of the white cock. A sick lion cures itself by eating an ape, eating one day and drinking the next; if the meat does not digest properly the lion pulls it out of its stomach with its claws. Lions are harmed by scorpions and killed by snakes.

When a lion is hungry it treats other animals with anger, leaping on them as it does on the ass. A hunting lion makes a circle with its tail around other animals, which do not dare to cross the line and so become its prey. The roar of a lion is alone enough to make other animals weak with fear.

Lions do not like to eat the previous day's prey, abandoning the remains of their last meal.

In Christian allegory, the three main natures of the lion each have a meaning. The lion erasing its tracks with its tail represents the way Jesus concealed his divinity, only revealing himself to his followers. The lion sleeping with its eyes open represents Jesus, physically dead after crucifixion, but still spiritually alive in his divine nature. The lion roaring over his dead cubs to bring them to life represents how God the father woke Jesus after three days in his tomb.

The other natures of the lion are taken as examples of how people are to live. Just as the lion will not attack a prostrate man, will allow captive men to depart, and is not easily angered, people should be slow to anger and quick to forgive.

The flesh of the lion is good for medicines, and its fat drives off serpents. Its fat mixed with Oleo rosaceo (rose oil?) is useful for treatment of skin conditions and heals burns. Its gall mixed with water clears and sharpens the sight, and can treat infections and epilepsy. Eating a lion's heart cures quartane fever.

Hildegard von Bingen says, "Draw off the skin of the lion, from the neck, over the head and its crown. Save it. If someone is mad from any infirmity in his head, he should warm up from having that fleece over his head, and he will recover his senses. One who ails from any other infirmity of the head should place that same skin from the lion’s head over his head, until his head warms up. He should soon take it off, and not let it lie on his head any longer, lest he be injured by its strength. He will be better. If someone is deaf, cut off a lion’s right ear. Another person should hold it on the ear of the deaf person, until the inside of his ear warms up from the ear of the lion, and no longer. He should say, 'Hear adimacus, by the living God, and by the sharpness of the lion’s strong hearing.' Do this often, and he will recover his hearing. One who is foolish will become wise for a long time if he places the dried heart of a lion on his breast for a little while—only until that spot becomes warm from it. If he were to allow it to remain longer, he would become nonsensical. ... Bury the heart of a lion in your house or anywhere you wish and, as long as it lies buried there, lightning will not cause fires in that place nor thunder crash there, for a lion is accustomed to roar when he hears thunder." (Throop translation)

The lion was one of the most popular animals in heraldry, though many of the heraldic animals taken as lions are actually intended as leopards. The attributes the lion was meant to represent in heraldry are similar to those given in the bestiary; the lion is noble, brave and fierce, but will only attack if attacked or in great need of food.

The lion was most often depicted rampant (standing upright on its hind legs, with its fore legs, claws extended, held in front of its chest), but sometimes passant (standing on all four feet or lying down). There were several varieties of lion pictured: ones with a forked tail (queue fourchée), the tail indicating the fierceness of the beast ('for when the Lion is wroth, first he beateth the earth with his taile, and afterwards as the wroth increaseth, he smiteth and beateth his own back'); with one body and two heads; with one head and two bodies (bicorporate), three bodies (tricorporate), four bodies (quadricorporate), or more; lions with wings, symbolic of the Christian concept of Resurrection; the Sea-Lion, not the natural sea lion but a beast with the head and mane of a lion, fore legs with webbed feet in place of claws, and a fish tail from the waist down, ending in whale-like flukes; and combinations such as a winged Sea-Lion.