Aspersion (la. aspergere/aspersio), in a religious context, is the act of sprinkling with water, especially holy water. Aspersion is a method used in baptism as an alternative to immersion or affusion. The word is formed of the Latin aspergere, 'to sprinkle', of ad, 'to', and spargo, 'I scatter'.

In addition, aspersion is performed as part of certain rites to remind people of their baptism, such as the renewal of baptismal vows performed by the Roman Catholic Church on Easter.

Aspersion is also a normal English usage, usually in the phrase "to cast aspersions upon…", to mean an attack on somebody's character or reputation.

St. Peter is said to have once baptized five thousand people in one day; this was most likely accomplished by aspersion or affusion. Baptism by affusion (pouring) was certainly in practice in Apostolic times, being prescribed by the Didache

but there are no accounts that clearly that show sprinkling rather than pouring was used.

The normal form of baptism for the first centuries—until at least the twelfth century—was immersion. However, when a person could not be immersed, baptism by aspersion or affusion was performed. There are records of people receiving baptism in prison, awaiting martyrdom, where immersion would be difficult or impossible, but the most common use was for a person who was ill and could not be removed from the bed; it therefore received the name "baptism of the sick." Because of its rarity, doubts arose about its validity, as is shown by St. Cyprian's affirming it in the face of questioning.