A post-office box or Post Office box (commonly referred to as a PO Box or a Postal Box) is a uniquely addressable lockable box located on the premises of a post office station.

In many countries, particularly in Africa, and the Middle East there is no 'door to door' delivery of mail. For example, should one post mail to a street address in Namibia, it will be returned to sender as undeliverable. Consequently renting a PO box has traditionally been the only way to receive mail in such countries, although some, like Jordan, are now introducing home delivery.

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 or CBD PO boxes are usually more expensive than a rural PO box.

In the US, 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.

A post office is a facility forming part of a postal system for the posting, receipt, sorting, handling, transmission or delivery of mail.

Post offices offer mail-related services such as acceptance of mail and sale of postage stamps, post office boxes, and sale of packaging and stationery. In addition, some post offices offer non-postal services such as passport applications and other government forms, car tax purchase, money orders, and banking services.

A post office may have a main customer service and point of sale area. Many postal codes or ZIP codes route an item to a specific post office; some correspond to a specific route or even delivery point (business, residence, or post office box).

In a "sorting office" or "delivery office", mail is sorted or processed for delivery. Large open spaces for sorting mail are also sometimes known as a sorting hall or postal hall. Over time, sophisticated mail sorting and delivery equipment has been developed, including Mail Rail.