In which country did Alan Turing earn his PhD?

Alan Turing earned his PhD in the United States.

Turing completed the doctorate at Princeton University in New Jersey in June 1938. He had gone to Princeton in 1936 to study under the mathematician Alonzo Church, whose work in mathematical logic strongly influenced Turing’s research. He spent much of the period from September 1936 to July 1938 in Princeton.

His dissertation, Systems of Logic Based on Ordinals, examined ordinal logic and introduced ideas about relative computation. It extended the theoretical work that had already made Turing famous for his 1936 paper on computable numbers and the concept now known as the Turing machine.

Turing was English and had studied at King’s College, Cambridge, so the United Kingdom is an understandable distractor. However, his doctorate was awarded in the United States, not Britain. He returned to the United Kingdom after completing the PhD and later became central to wartime codebreaking at Bletchley Park.