A post box (British English; also written postbox; also known as pillar box), also known as a collection box, mailbox, letter box or drop box (American English), is a physical box into which members of the public can deposit outgoing mail intended for collection by the agents of a country's postal service. The term post box can also refer to a private letter box for incoming mail.

In 1653, the first post boxes are believed to have been installed in and around Paris. By 1829, post boxes were in use throughout France. The first public post boxes in Poland were installed in Warsaw in 1842.

A post box originally installed in the wall of the Wakefield Post Office is dated 1809 and is believed to be the oldest example in Britain. It is now on display at the Wakefield Museum.

In Britain, the first red pillar postboxes were erected in Guernsey in 1852. Roadside wall boxes first appeared in 1857 as a cheaper alternative to pillar boxes, especially in rural districts. In 1853 the first pillar box in the United Kingdom was installed at Botchergate, Carlisle. In 1856, Richard Redgrave of the Department of Science and Art designed an ornate pillar box for use in London and other large cities. In 1859 the design was improved, and this became the first National Standard pillar box. Green was adopted as the standard colour for the early Victorian post boxes. Between 1866 and 1879 the hexagonal Penfold post box became the standard design for pillar boxes and it was during this period that red was first adopted as the standard colour. The first boxes to be painted red were in London in July 1874, although it would be nearly 10 years before all the boxes had been repainted. In 2012, to celebrate Olympic gold medals for Team GB, selected boxes were painted gold.

The first public letter boxes (post boxes) in Russia appeared in 1848 in St. Petersburg. They were made of wood and iron. Because these boxes were lightweight and easily stolen, they disappeared frequently; later boxes were made of cast iron and could weigh up to 45 kilograms (99 lb).^{[citation needed]}

In March 2025, Post Danmark announced it was quitting the letter delivery business at the end of 2025, and as so remove all the post boxes in Denmark. The 1200 post boxes in question would be sold off to collectors, with the money going charity.

The post box arrived in the late 19th century Hong Kong and were made of wood. In the 1890s, metal pillar box appeared in Hong Kong and remained in use until the late 1990s. From the 1890s to 1997 the boxes were painted red and after 1997 were painted green.

The United States Post Office Department began installing public mail collection boxes in the 1850s outside post offices and on street corners in large Eastern cities. American collection boxes were initially designed to be hung or supported, and were mounted on support pillars, lamp-posts, telegraph poles, or even the sides of buildings. By the 1880s, these pillar boxes were made of heavy cast iron to deter theft or vandalism. As mail volume grew, the Post Office Department gradually replaced pillar mailboxes with larger free-standing models, though many of the pillar boxes continued in service as late as the 1960s.