DragonFly BSD is a free Unix-like operating system created as a fork of FreeBSD 4.8. Matthew Dillon, an Amiga developer in the late 1980s and early 1990s and a FreeBSD developer between 1994 and 2003, began work on DragonFly BSD in June 2003 and announced it on the FreeBSD mailing lists on July 16, 2003.

Dillon started DragonFly in the belief that the methods and techniques being adopted for threading and symmetric multiprocessing in FreeBSD 5 would lead to poor system performance and cause maintenance difficulties. He sought to correct these suspected problems within the FreeBSD project. Due to ongoing conflicts with other FreeBSD developers over the implementation of his ideas his ability to directly change the FreeBSD code was eventually revoked. Despite this, the DragonFly BSD and FreeBSD projects still work together contributing bug fixes, driver updates and other system improvements to each other.

Intended to be the logical continuation of the FreeBSD 4.x series, DragonFly's development has diverged significantly from FreeBSD's, including a new Light Weight Kernel Threads implementation (LWKT), a light weight ports/messaging system, and feature-rich HAMMER file system. Many concepts planned for DragonFly were inspired by the AmigaOS operating system.