FreeBASIC is a free/open-source, QuickBASIC compiler for Microsoft Windows, DOS, and Linux.

When used in its "QB" language mode, FreeBASIC provides a high level of support for programs written for QuickBASIC. Many programs written for QuickBASIC will compile and run in this mode with no changes needed. However, for compilation in the FreeBASIC default language mode, most substantial programs will require changes.

FreeBASIC is a self-hosting compiler that uses the GNU Binutils programming tools as backends and can produce console, graphical/GUI executables, dynamic and static libraries. FreeBASIC fully supports the use of C libraries and has partial C++ library support. This lets programmers use and create libraries for C and many other languages. It supports a C-style preprocessor capable of multiline macros, conditional compiling, and file inclusion.

FreeBASIC has been rated close in speed with mainstream tools, such as GCC.