J.R.R. Tolkien’s academic field was philology.
Philology is the historical study of languages and texts, especially how languages develop and how written sources preserve earlier forms. Tolkien specialized in Germanic languages and medieval literature, including Old English, Middle English, Old Norse, and Gothic.
His professional expertise directly influenced his fiction. He created languages with histories, sound patterns, and relationships, then developed peoples and legends around them. His approach to invented language was therefore closely connected to his scholarly interest in how real languages change over time.
Tolkien is also often described as a medievalist, linguist, or literary scholar. Those descriptions are relevant, but philologist is the traditional term most closely associated with his academic career. Astronomy, botany, and geology were not his professional disciplines.