The Final Unfinished Voyage of Jack Aubrey is the unfinished twenty-first historical novel in the Aubrey-Maturin series by English author Patrick O'Brian, first published in its incomplete form in 2004. It appeared in the United States of America under the title of 21.

The Aubrey–Maturin series is a sequence of nautical historical novels—20 completed and one unfinished—by English author Patrick O'Brian, set during the Napoleonic Wars and centring on the friendship between Captain Jack Aubrey of the Royal Navy and his ship's surgeon Stephen Maturin, a physician, natural philosopher, and intelligence agent. The first novel, Master and Commander, was published in 1969 and the last finished novel in 1999. The 21st novel of the series, left unfinished at O'Brian's death in 2000, appeared in print in late 2004. The series received considerable international acclaim, and most of the novels reached The New York Times Best Seller list. These novels comprise the heart of the canon of an author often compared to Jane Austen, C. S. Forester and other British authors central to English literature.