The Thief 3 Editor is derived from the Unreal Editor, with which the user can create, edit, test and publish his own missions, through the construction of maps, scripts, static meshes, sound schemas, textures, skeletal meshes, skeletal animations, books and videotextures.

This editor faced an unfortunate history when shortly after its release many fans tried to implement gameplay elements from the first two Thief games that were not present in Thief: Deadly Shadows. As this failed in several instances, T3Ed was often abandoned despite its ability to create fan missions in the style of TDS, even exceeding them in size and possibilities (like complex scripting, custom objects and textures, cubemaps etc.). With the Dark Mod publicly presented as a can-do alternative, T3Ed suffered from all this and the negative impression that TDS left on many fans. Hence its editing community remains small.

The actual major weaknesses of T3Ed, aside from certain gameplay elements like swimmable water and rope arrows (to which only bulky workarounds have been presented as yet) are its inappropriate tendency to display large numbers of dynamic shadow-casting lights in a small area and the difficulties of finding the right pieces of software to include custom objects or textures (although a workaround for textures does exist). The most severe flaw, however, is the great number of undeveloped or inoperable functions in its menus, and the many bugs which can cause a crash.

Despite these glaring faults, T3Ed has many useful features: The triggerscript system, although limited and bug-riddled, allows access to many aspects of the game's mechanics in a simple condition-action way. Complex patrol routes can be designed quickly and easily. The vast object database is easy to operate and can be used to re-skin static meshes with a single mouse click. High-resolution textures, normalmaps, cubemaps, coloured lights, texture projecting lights, editable emitters, and other design features provide a great deal of variation and control over visual effects.

After the creation of one or more missions, the user must include any relevant files in a zip archive, so that is readable by the GarrettLoader (polyhedric utility to load fan missions of multiple kind), or create an executable which can install and uninstall all new files in the game. New mission loading solutions like ModernLoader or Thief Fan Mission Manager work by linking the original game directory and setting up the FM as a setup of its own. More modern techniques are still being developed. The editor for Thief: Deadly Shadows can be downloaded here.

The unit of measure in the editor is the Unreal Unit (UU), which is approximated to be 1/64 of a DromEd Unit, which is circa 10 inches (1uu=.625in, 1in=1.6uu, 1ft=19.2uu).