Basel ( /ˈbɑːzəl/) or Basle ( /ˈbɑːl/; German: Basel; pronounced [ˈbaːzəl]) is Switzerland's third most populous city with about 166,000 inhabitants. Located where the Swiss, French and German borders meet, Basel also has suburbs in France and Germany. With 830,000 inhabitants in the tri-national urban agglomeration as of 2004, Basel is Switzerland's second-largest urban area.

Located in northwest Switzerland on the river Rhine, Basel functions as a major industrial centre for the chemical and pharmaceutical industry. The Basel region, culturally extending into German Baden-Württemberg and French Alsace, reflects the heritage of its three states in the modern Latin name: "Regio TriRhena". It has the oldest university of the Swiss Confederation (1460). Basel is German-speaking. The local variant of the Swiss German dialects is called Basel German.

Basel is among the most important cultural centres of Switzerland. The city comprises a large number of theatres and many museums, including the Museum of Fine Arts, the world's oldest art collection accessible to the public. In addition the Theater Basel was chosen in 1999 as the best stage for German-language performances and in 2009 & 2010 as "Opera of the Year" by German Opera Magazine "Opernwelt".