The Lost Symbol is a 2009 novel written by American writer Dan Brown. It is a thriller set in Washington, D.C., after the events of The Da Vinci Code, and relies on Freemasonry for both its recurring theme and its major characters.

Released on September 15, 2009, it is the third Brown novel to involve the character of Harvard University symbologist Robert Langdon, following 2000's Angels & Demons and 2003's The Da Vinci Code.