A safe deposit box (or safety deposit box) is a type of safe usually located in groups inside a vault or in the back of a bank or post office. It usually holds things such as valuable jewels, excess money, or important documents such as wills or property deeds that a person might feel afraid to leave at home due to fear of theft, fire, or flood. In the typical arrangement, a renter pays the bank a fee for the use of the box, which can be opened only with production of the assigned key, the bank's master key, the proper signature, or perhaps a code of some sort.

Because the contents of the box may be a mystery, safety deposit boxes can serve as useful plot devices in works of fiction, particularly thrillers. For example, in The Da Vinci Code and The Bourne Identity, entire sections of the storyline revolve around control of a safety deposit box.