Monaco, officially the Principality of Monaco, is a sovereign city-state and microstate in Western Europe. Situated on the French Riviera, it is a semi-enclave bordered by France to the north, east, and west, with the Mediterranean Sea to the south; the Italian region of Liguria is about 15 km (9.3 mi) east. With a population of 38,423 living in an area of 2.08 km² (0.80 sq mi), Monaco is the second smallest sovereign state in the world, after Vatican City, as well as the most densely populated. It also has the world's shortest national coastline of any non-landlocked nation, at 3.83 km (2.38 mi). Fewer than 10,000 of its residents are Monégasque nationals. While the official language of Monaco is French, Italian and Monégasque are also widely spoken and understood.

Monaco is governed under a form of semi-constitutional monarchy, with Prince Albert II as head of state, who holds substantial political powers. The prime minister, who is the head of government, can be either a Monégasque or French citizen; the monarch consults with the Government of France before an appointment. Key members of the judiciary are detached French magistrates. The House of Grimaldi has ruled Monaco, with brief interruptions, since 1297. The state's sovereignty was officially recognised by the Franco-Monégasque Treaty of 1861, with Monaco becoming a full United Nations voting member in 1993. Despite Monaco's independence and separate foreign policy, its defence is the responsibility of France, notwithstanding two small military units.

Monaco is recognised as one of the wealthiest and most expensive places in the world. Its economic development was spurred in the late 19th century with the opening of the state's first casino, the Monte Carlo Casino, and a rail connection to Paris. The country's mild climate, scenery, and gambling facilities contributed to its status as a tourist destination and recreation centre for the wealthy. Monaco has become a major banking centre and sought to diversify into the services sector and small, high-value-added, non-polluting industries. Monaco is a tax haven; it has no personal income tax (except for French citizens) and low business taxes. Over 30% of residents are millionaires, with real estate prices reaching €100,000 ($116,374) per square metre in 2018. Monaco is a global hub of money laundering, and in June 2024 the Financial Action Task Force placed Monaco under increased monitoring to combat money laundering and terrorist financing.

Monaco is among the 46 Member States which constitute the Council of Europe. It is not part of the European Union (EU), but participates in certain EU policies, including customs and border controls. Through its relationship with France, Monaco uses the euro as its sole currency. Monaco joined the Council of Europe in 2004 and is a member of the Organisation internationale de la Francophonie (OIF). It hosts the annual motor race, the Monaco Grand Prix, one of the original Grands Prix of Formula One. The local motorsports association gives its name to the Monte Carlo Rally, hosted in January in the French Alps. The principality has a club football team, AS Monaco, which competes in French Ligue 1 and has been French champions on multiple occasions, as well as a basketball team, which plays in the EuroLeague. Monaco is a centre of marine conservation and research, being home to one of the world's first protected marine habitats, an Oceanographic Museum, and the International Atomic Energy Agency Marine Environment Laboratories, the only marine laboratory in the UN system.

Monaco was first mentioned by the Greek historian Hecataeus of Miletus, who referred to it as "Monoikos, a Ligurian town". Strabo referred to it as Monoikon limen, explaining that Monoikos (Greek for "the solitary one") was another name for Heracles, who had a temple here; it is likely, however, that it is in fact an alteration of an older Ligurian name.

In 1191, the Holy Roman Empire granted Monaco to the Genoese in return for cracking down on pirates. By the early 1200s, the Genoese had built a castle on the rock and were utilizing the port. An ousted branch of a Genoese family, the Grimaldis, captured it in 1297 by posing as monks, but then had to contest it for a century before gaining official control. Though the Republic of Genoa would last until the 19th century, it allowed the Grimaldi family to keep Monaco. Likewise, both France and Spain left it alone for hundreds of years due to agreements with either of them, especially for defense. France annexed it in the French Revolution at the end of the 18th century, but after the defeat of Napoleon it was put under the care of the Kingdom of Sardinia.

In the 19th century, when Sardinia became a part of Italy, the region came under French influence but France allowed it to remain independent and it escaped being incorporated into Italy. However, it shrank considerably when it traded two nearby towns in exchange for sovereignty from France. Monaco relied on tourism from the late 19th century to remain financially solvent, and it was at this time the famous casino and hotels were established. Monaco was overrun by the Axis powers in the 1940s during the Second World War and for a short time was administered by Italy, then Nazi Germany, before being liberated. Although the occupation lasted for just a short time, it resulted in the deportation of the Jewish population and execution of several French Resistance members from Monaco. Since then Monaco has been independent. It has taken some steps towards integration with the European Union.

Following a grant of land from Emperor Henry VI in 1191, Monaco was refounded in 1215 as a colony of Genoa. Monaco was first ruled by a member of the House of Grimaldi in 1297, when Francesco Grimaldi, known as "Malizia" (translated from Italian either as "The Malicious One" or "The Cunning One"), and his men captured the fortress protecting the Rock of Monaco while dressed as Franciscan friars – a monaco in Italian – although this is a coincidence as the area was already known by this name.