A motu proprio (Latin for: "on his own impulse") is a document issued by the Pope (or by a monarch) on his own initiative and personally signed by him.

When issued by the Pope, a motu proprio may be addressed to the whole Church, to part of it, or to some individuals. A document issued motu proprio has its legal effect even if the reasons given for its issuance are found to be false or fraudulent, a fact which would normally render the document invalid. Its validity is based on its issuance by the pope by his own initiative, not upon the reasons alleged.