Fifth Business is a 1970 novel by Canadian novelist, playwright, critic, journalist, and professor Robertson Davies.

Fifth Business is perhaps Davies' best-known novel, and many considered it his finest. It is the first installment of the Deptford Trilogy and is a story of the life of the narrator, Dunstan Ramsay.

Ramsay's avid interest in hagiology and his guilty connection to Mary Dempster provide most of the impetus and background for this novel. He spends much of the book struggling with his image of Mary Dempster as a fool-saint and dealing with issues of guilt that spawned from an accident in which he was involved as a child.

Davies employs many different techniques in the writing of Fifth Business. He discusses many themes in the novel, perhaps the most important being the difference between materialism and spirituality. By spirituality, Davies discusses how religion is not necessarily integral to the idea — demonstrated by the corrupt Reverend Leadbeater who reduces the Bible to mere economic terms.

Davies, being an avid follower of Carl Jung's ideas, also employs them in Fifth Business. Characters are clear examples of Jungian archetypes and events are demonstrative of Jung's idea of synchronicity.

The entire story is told in the form of a letter written by Ramsay on the occasion of his retirement as master at Colborne College and addressed to the school Headmaster.

Ramsay's life bears many similarities to Davies', and so one may find Fifth Business to be semi-autobiographical in nature. Davies projected his life experiences into many of his works and so one may find it no surprise that he chose to do so again in Fifth Business. Davies thought of it as "autobiographical, but not as young men do it; it will be rather as Dickens wrote David Copperfield, a fictional reworking of some things experienced and much re-arranged." Davies allows us to peer through a window into his childhood in Thamesville, Ontario and through his young life into higher education and beyond through the character of Ramsay and throughout the Deptford trilogy. Davies provides us, in Fifth Business, with an autobiography which is "not a sweating account of the first time he backed a girl into a corner", but an account of his spirit, his memories, and his deeper life experiences.

First published by Macmillan of Canada in 1970, Fifth Business was selected 40th on the American Modern Library's "reader's list" of the 100 best novels of the 20th century.