What is the only country that borders the United Kingdom?

The Republic of Ireland.

The only country that shares a land border with the United Kingdom is the Republic of Ireland. This border is with Northern Ireland, which is part of the United Kingdom. The Republic of Ireland is a separate sovereign nation. The border between Northern Ireland and the Republic of Ireland is the only land border for the United Kingdom, since England, Scotland, and Wales are all located on the island of Great Britain and are surrounded by water.