M. Rex (short for Machina Rex) was a title by Image Comics' subsidiary Avalon Comics. It began in 1999. It inspired the cartoon series Generator Rex. It ran for two issues.

M. rex may refer to:

• Megalopanax rex, a flowering plant species endemic to Cuba
• Melanocharacidium rex, a species of South American darter
• Meriones rex, the king jird, a rodent species found in Saudi Arabia and Yemen
• Mormula rex, a species of sea snail
• Mylomys rex, the Ethiopian mylomys, a rodent species found only in Ethiopia
• Myodes rex, the Hokkaido red-backed vole, a rodent species found only in Japan
• M. Rex, a comic book series that inspired Generator Rex.