A post office box (commonly abbreviated as P.O. box, or also known as a postal box) is a uniquely addressable lockable box located on the premises of a post office.

In some regions, particularly in Africa, there is no door to door delivery of mail; for example, in Kenya. Consequently, renting a PO box has traditionally been the only way to receive mail in such countries.

Generally, post office boxes are rented from the post office either by individuals or by businesses on a basis ranging from monthly to annual, and the cost of rent varies depending on the box size. Central business district (CBD) PO boxes are usually more expensive than rural PO boxes.

In the United States, the rental rate used to be uniform across the country. Now, however, a postal facility can be in any of seven fee groups by location; in addition, certain customers qualify for free box rental, usually because the Postal Service does not offer carrier-route delivery to their physical addresses.

In the United Kingdom, Royal Mail PO boxes are often little more than pigeon-holes in the secure section of a sorting office and are accessible only by staff. In such cases, the renter of the PO box will be issued with a card showing the PO box number and delivery office name and must produce this to the desk staff when collecting mail. For an additional fee, the Royal Mail will deliver received items to the renter's geographical address.

Some private companies (e.g., United Parcel Service and commercial mail receiving agencies) offer similar services of renting a mailbox in a public location. The difference is that mail sent there is addressed to a street address (along with the private box number), instead of just addressed to "PO Box CSX". Virtual mailboxes are similar to P.O. boxes but usually provide additional services such as scanning or forwarding.