A piru is a fiend or demon in Finnish mythology. The word is inherited from Proto-Finnic *piru* ‘devil, demon’, with cognates in Karelian, Ingrian and Estonian; its ultimate origin is unknown.

In folklore, a piru is often featured as a nasty spirit of the forest with which a smart aleck either wins or loses a battle of wits, giving or receiving a forfeit in return. In many cases, poltergeist and haunting phenomena are described as "pirus". The Devil may be referred to as (proper noun) Piru, or Pääpiru, the main piru.^{[citation needed]}

"Piru" is also a mild swearword in Finnish.