Calvin's Case (1608), 77 ER 377, (1608) Co Rep 1a, also known as the Case of the Postnati, was a 1608 English legal decision establishing that a child born in Scotland, after the Union of the Crowns under King James VI and I in 1603, was considered under the common law to be an English subject and entitled to the benefits of English law. Calvin's Case was eventually adopted by courts in the United States, and the case played an important role in shaping the American rule of birthright citizenship via jus soli.

The Union of the Crowns was the accession of James VI of Scotland to the throne of the Kingdom of England as James I and the practical unification of some functions of the two separate realms under a single individual on 24 March 1603. It followed the death of James's cousin, Elizabeth I of England, the last monarch of the Tudor dynasty.