5 non-EU countries that officially use the euro as currency

The euro represents much more than just the common currency of the European Union. Given that the euro has, in total value of circulating banknotes and coins, the largest combined value of cash of any single currency (having surpassed the United States dollar) and the largest number of coins valued at one euro or more being foreign within the EU, it has been taken up extensively outside the EU. It is issued by the European Central Bank and exists in popular denominations of banknotes: €5, €10, €20, €50 and €100; however, the denominations of €200 and €500 are used far less than the others. Euro coins are produced in denominations of 1 cent, 2 euro.
All 20 EU member states that have adopted the euro as their currency form one part of the European Union; non-EU countries such as Kosovo and Montenegro have also since adopted it. Some do this through mutual deals with the European Union, while others have made a unilateral move. Here is a list of five such countries that use the euro, despite not being members of the European Union. Data sourced from World Population Review.

Andorra, a tiny country between France and Spain that is not a member of the European Union, but it has officially adopted the euro. This was made possible after the country concluded a formal monetary agreement with the EU in 2006 – enabling it to use the euro as legal tender and mint its own euro coins.
Andorra acceded to the United Nations and Council of Europe in 1993. Despite its small size, it maintains an active international presence and gained global recognition in 2004 when the Madriu-Perafita-Claror Valley was declared a UNESCO World Heritage Site for its historic pastoral landscape and many other things.

Monaco, a sovereign city-state on the French Riviera, relies on the euro as its official means of currency through an agreement with France. Monaco is not formally a part of the European Union (EU), but it participates in certain EU policies, including customs and border controls.
The principality became a member of the Council of Europe in 2004 and is part of the Organisation internationale de la Francophonie. Monaco’s financial relationship with France goes back generations, and French customs laws apply in Monaco and its adjacent waters. The euro is now a part of normal life on the island today.

Montenegro is one of the most peculiar examples of euro adoption. The nation is not part of the European Union or the eurozone, and has no formal monetary agreement with other EU countries. Still, it has effectively adopted the euro as its domestic currency in legal terms since 2002.
Montenegro has had several currencies before officially adopting the euro, which included two versions of the Montenegrin perper, during the first half of the 20th century and second half of the 20th century, and also held a parallel use of Yugoslavia’s dinar after being part of Yugoslav since its de facto independence. Today, Montenegro’s financial system is based on the euro – even with its special unilateral position.

The Republic of San Marino, which claims to be the world's oldest republic still in existence, changed over from its lira coins after 1 January 2002. Prior to that date, the Sammarinese lira was pegged to, and exchangeable with, the Italian lira (at par) and both circulations were accepted.
Not a member of the European Union, San Marino has close economic links to Italy. Today, its official currency is the euro and it mints its own euro coins—coins popular with collectors throughout Europe.

The euro has been used in Vatican City since 2002, making it the most easily adopted foreign currency. Before that it used the Vatican lira, which was not a currency of its own but rather a version of the Italian lira minted specifically for the city-state by the Banca d’Italia.
After the integration, Vatican City started to use one of its own euro coins while selling several others. It is landlocked within Rome and so not a member of the European Union; but the euro lies at the heart of its financial system.