In the United Kingdom and Republic of Ireland, a county town is the most important town or city in a county. It is usually the location of administrative or judicial functions within a county and the place where the county's members of Parliament are elected. Following the establishment of the English county councils in 1889, the headquarters of the new councils were usually located in the county town of each county. However, the concept of a county town pre-dates the establishment of these councils.

The concept of a county town is ill-defined and unofficial. Some counties have their administrative bodies located elsewhere. For example, Lancaster is the county town of Lancashire, but the county council is located in Preston. Some county towns are no longer situated within the administrative county because of changes in the county's boundaries. For example, Nottingham is administered by a unitary authority separate from the rest of Nottinghamshire.