In law, motu proprio (Latin for "on his own impulse") describes an official act taken without a formal request from another party. Some jurisdictions use the term sua sponte for the same concept.

In Catholic canon law, it refers to a document issued by the pope on his own initiative and personally signed by him. Such a document 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.

The first motu proprio was promulgated by Pope Innocent VIII in 1484. It continues to be a common form of papal rescript, especially when establishing institutions, making minor changes to law or procedure, and when granting favours to persons or institutions.