Monaco, officially the Principality of Monaco, is a sovereign city-state and microstate on the French Riviera a few kilometres west of the Italian region of Liguria, in Western Europe, on the Mediterranean Sea. It is a semi-enclave bordered by France to the north, east and west. The principality is home to 38,682 residents, of whom 9,486 are Monégasque nationals; it is recognised as one of the wealthiest and most expensive places in the world. The official language is French; Monégasque, English and Italian are spoken and understood by many residents.

With an area of 2.08 km2 (0.80 sq mi), Monaco is the second-smallest sovereign state in the world, after Vatican City. Its population of 38,367 in 2023 makes it the most densely populated sovereign state. Monaco has the world's shortest coastline: 3.83 km (2.38 mi). The principality is about 15 km (9.3 mi) from the border with Italy and consists of nine administrative wards, the largest of which is Monte Carlo.

The principality is governed under a form of constitutional monarchy, with Prince Albert II as head of state, who wields political power despite his constitutional status. 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, besides maintenance of two small military units.

Monaco's 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. Monaco's mild climate, scenery, and gambling facilities have contributed to its status as a tourist destination and recreation centre for the rich. 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 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 been French champions on multiple occasions, and a basketball team, which plays in the EuroLeague. A centre of research into marine conservation, Monaco is 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 structure.