Which Oxford college did J.R.R. Tolkien teach at as a professor and fellow?

J.R.R. Tolkien taught at and became a fellow of Merton College, Oxford.

Tolkien moved to Merton in 1945, when he became the Merton Professor of English Language and Literature. He remained associated with the college until his retirement in 1959. This was the final major Oxford post in a career that had also included teaching at the University of Leeds and Pembroke College, Oxford.

His Merton years overlapped with the period in which he completed and published The Lord of the Rings. Tolkien’s academic work focused on language, medieval literature, and philology, fields that strongly influenced the invented languages, names, and historical depth of his fiction.

The college is sometimes confused with Pembroke College because Tolkien was a fellow and professor there earlier in his Oxford career. Pembroke was associated with his first Oxford professorship; Merton was his later home and the institution named in this question.