Which novel did J.R.R. Tolkien publish first, in 1937?

J.R.R. Tolkien first published The Hobbit in 1937.

The book appeared on 21 September 1937 under the title The Hobbit, or There and Back Again. It follows Bilbo Baggins, a hobbit drawn into a journey with Thorin Oakenshield and a company of Dwarves. The adventure eventually brings Bilbo into contact with Gollum and the One Ring.

The Hobbit began as a children’s fantasy novel, but its setting and mythology connected it to the larger legendarium Tolkien had been developing for years. Its success encouraged Tolkien’s publisher to request a sequel, which ultimately became The Lord of the Rings.

The Lord of the Rings was published later, in three volumes from 1954 to 1955. The Silmarillion was published posthumously in 1977, edited by Christopher Tolkien, so neither is the answer to the question about Tolkien’s first published novel.