The postal counties of the United Kingdom, now known as former postal counties, were postal subdivisions in routine use by the Royal Mail until 1996. The purpose of the postal county – as opposed to any other kind of county – was to aid the sorting of mail by differentiating between similar post towns. Since 1996 this has been done by using the outward code (first half) of the postcode instead. For operational reasons, the former postal counties, although broadly based on the counties of the United Kingdom, did not match up with their boundaries; in some cases there were significant differences. The boundaries changed over time as post towns were created or amended.

According to the Royal Mail, the former postal county data no longer forms part of postal addresses. It was removed from the Postcode Address File database in 2000 and does not form part of its code of practice for changing addresses. Despite this, county data is routinely sold to companies, ostensibly to let them cleanse their own address data. As the former postal county data was the last to be in routine use, some organisations have continued to use this obsolete data as part of postal addresses. In 2009, the Royal Mail code of practice consultation included discussion of the possible replacement of the currently supplied "alias data" with an up-to-date county information data field. In 2010, the regulator advised Royal Mail to cease supply of county data altogether, and a timetable was put in place for this to occur between 2013 and 2016.

In the 19th century and early 20th centuries, the Post Office required the name of the post town to be included in addresses, but advised against including the name of the county. The exception was if a post town's name was not unique, in which case the county name was to be included in the address in order to distinguish between post towns of the same name.

Sometime between 1920 and 1934 the Post Office changed its advice on counties. It recommended instead that the name of the county in which the post town lay should be included in most cases, with the exception of certain large towns and cities and those which gave their name to the county.

As the name of the county to be used in the postal address was derived from the post town, there were various scenarios where there were differences between the postal county and the geographic or administrative counties:

First, many of the approximately 1,500 post towns straddled county boundaries, and the postal addresses of all places in such areas included the postal county of the post town, regardless of their actual location. In a written answer in the House of Lords in 1963, Lord Chesham, Parliamentary Secretary at the Ministry of Transport estimated that about 7% of towns and villages with a post office lay in a postal county different from their geographical county. He went to explain that:

Postal addresses are in effect routing instructions for Post Office sorters and, in settling what they should be, the main concern is to ensure a quick and efficient service at reasonable cost. The general aim is to align postal boundaries with those of the counties and where this has not been done it is usually because road and rail communications are such that mail can be got more quickly and efficiently to and from certain villages, et cetera, via a neighbouring county than via the county in which they are situated. Alignment of postal and county boundaries in these instances would mean either a poorer postal service for the villages, et cetera, in question or prohibitively heavy additional costs.

— John Cavendish, 5th Baron Chesham, Parliamentary Secretary at the Ministry of Transport written answer to Parliamentary question