FCEUX (Family Computer Emulator Ultra X) is a NES/Famicom/FDS emulator. It is an evolution of FCE Ultra, merging elements of FCEU's many branches into a single application. FCE Ultra is itself based on Bero's Family Computer Emulator.

It is a popular choice for tool-assisted speedruns. The alternative, BizHawk, has more accurate emulation, but both are acceptable.

A special BIOS file named disksys.rom is required to play FDS files (files ending in .fds). It should be placed in the FCEUX folder.

FCEUX can convert .fcm movies to .fm2 in the Tools menu. Most movies will sync with FCEUX when converted. Details on movies that do or don't convert are documented here.

FCEUX has full support of rerecording, frame advance, speed slowdown, display (movie count, input), fast-forward, memory watch, and Lua scripting. All keymaps are customizable.

Older versions of FCEU (0.98.28) allow enabling background input as well as recording from reset. They have been taken out in FCEUX.