In computer file systems, the root directory is the first or top-most directory in a hierarchy. It can be likened to the root of a tree — the starting point where all branches originate.

To use the example of a physical file cabinet, if the separate drawers in the file cabinet are represented as the highest level of sub-directories in the file system or system prompt, then the room the file cabinet is in, may be represented as the root directory. That is, the other directories may be inside it, but the root directory cannot go in any other directories, at least in that file system. In most operating systems, files can be placed inside the root directory, as well in its sub-directories. One may envision this as placing paper files anywhere in the room, or into any file cabinet within the room.

Unix abstracts the nature of this tree hierarchy entirely, and in Unix and Unix-like systems, the root directory is denoted /. Though the root directory is conventionally referred to as /, the directory entry itself has no name — its name is the "empty" part before the initial directory separator character (/). All filesystem entries, including mounted partitions are "branches" of this root.