J.R.R. Tolkien’s academic friend and fellow fantasy author who wrote The Chronicles of Narnia was C. S. Lewis.
Tolkien and Lewis were colleagues and friends at Oxford, where they participated in literary discussion groups including the Inklings. Their conversations involved mythology, language, literature, and Christian themes. Lewis’s The Chronicles of Narnia became one of the most influential fantasy series for children, while Tolkien was developing and revising Middle-earth.
The two authors influenced one another, though their careers and fictional worlds remained separate. Lewis encouraged Tolkien’s work and read portions of Tolkien’s writing, including material connected with The Lord of the Rings. Tolkien, in turn, read and commented on some of Lewis’s fiction.
A common mix-up is to identify Lewis as a co-author of Tolkien’s books. He was not. Lewis created Narnia, while Tolkien created Middle-earth. Their association was a literary friendship, not a shared authorship arrangement.