Persona non grata (plural personae non gratae), literally "an unwelcome person," is a term generally reserved for diplomats. The term is more widely used than its narrow legal definition, but use in non-diplomatic cases is, essentially, an affectation.

Under the Vienna Convention on Diplomatic Relations Article 9 a receiving State may "at any time and without having to explain its decision," declare any member of a diplomatic staff as persona non grata - that is not acceptable (while a persona grata is acceptable) - even before they arrive in the State. It is usual for a person so declared to be recalled to their home nation. If they are not recalled the receiving State "may refuse to recognize the person concerned as a member of the mission."

While diplomatic immunity protects mission staff from civil and criminal laws, depending on rank, under Articles 41 and 42 of the Vienna Convention they are bound to respect national laws and regulations (amongst other issues). Breaches of these articles can lead to persona non grata being used to 'punish' erring staff. It is also used to expel diplomats suspected of espionage, or as a symbolic indicator of displeasure (e.g. the Italian expulsion of the Egyptian First Secretary in 1984). So-called "tit-for-tat" exchanges have occurred, notably during the Cold War.