This is a list of countries that have a land border with only one other country. Some on this list have a maritime border with additional countries. Some countries, which are not listed here, have no land border but do have a maritime border with a single other country, such as Sri Lanka.
There are generally three arrangements by which a country would have a single land border:

an island divided between two states, such as between Haiti and the Dominican Republic, or between Ireland and the United Kingdom.
a peninsula or a semi-enclave, where one country has a land border with a neighbouring one but is otherwise surrounded by sea, while the neighbour borders other countries—examples are Portugal (neighbouring Spain), The Gambia (surrounded by Senegal) and Brunei (surrounded by Malaysia). This applies to peninsulas, but also to non-peninsular nations like The Gambia and Brunei.
the three countries forming a landlocked true enclave, surrounded by a larger country: San Marino and Vatican City (within Italy) and Lesotho (within South Africa).Territory leased or ceded by one country to another for perpetual use, but not in sovereignty, such as Guantanamo Bay Naval Base in Cuba, or memorials, such as the American Cemetery in France, do not constitute true territorial borders because the land occupied remains a formal part of the host country.
This list is based on the Correlates of War Direct Contiguity data set, with maritime causeways and bridges not being counted.

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