The Patronus Charm is a very difficult spell featured in the Harry Potter series of fantasy novels by J. K. Rowling. It is the only known means of defence against the horrid Dementors.

The Charm creates a projection of the caster's most positive feelings, like happiness, hope and the desire to survive, known as the Patronus. In essence, a Patronus is the exact opposite of a Dementor. Even though it embodies all the good emotions a Dementor feeds on, a Patronus lacks despair by definition, which renders it immune and a weapon rather than a feast. It can shield its caster from Dementors and even drive them away.

When cast, a Patronus appears silvery, insubstantial and semi-transparent. Less advanced ones are momentary formless bursts, like mist or water vapour. A fully-fledged (or corporeal) Patronus takes a fixed animal shape that depends on the person. Harry Potter's Patronus is a stag, his father's Animagus form. Hermione Granger's is an otter, her favourite animal. The animal obeys the conjurer and fades away shortly after its use ends.

The Patronus Charm is simple in theory, but hard in practice. The technical requirements are the incantation Expecto patronum (presumed dog latin for I call/summon the protector) and a wand. Unfortunately, the caster must focus very strongly on his or her positive emotions in order to draw from them. This is not easy at the best of times, let alone when actually in the presence of happiness-sucking Dementors. The best way is to concentrate on a very happy thought; usually golden memories. Harry has used both memories (such as the moment he learnt of his heritage and Hogwarts) and wishes (such as Dolores Umbridge being fired).

According to Fantastic Beasts and Where to Find Them, the Patronus Charm is also the only known spell that can deal with shroud-like Lethifolds, creatures not found in the books' setting that silently slip into houses to suffocate and digest their victims. When not dealing with vile creatures, a Patronus is pretty but rather useless, although they may have something to do with the communication methods of the Order of the Phoenix that are yet to be revealed.