Digital Fortress is a techno-thriller novel by bestselling author Dan Brown, first published in 1998 by St. Martin's Press. The story follows Susan Fletcher, a head cryptographer at the National Security Agency, when the NSA's invincible code-breaking machine encounters a mysterious code it cannot break. Caught in a web of secrecy and lies, Fletcher races from the underground hallways of power to the skyscrapers of Tokyo and the towering cathedrals of Spain to prevent the biggest intelligence disaster in U.S. history.

The novel explores themes of personal freedom versus national security, blending real-world cryptographic concepts with fast-paced suspense. Digital Fortress was Dan Brown's debut novel and helped establish him as one of the most popular thriller writers in the world, preceding his blockbuster hit The Da Vinci Code. Available in paperback, e-book, and audiobook formats.