Kaunas (/ˈkaʊnəs/; Lithuanian: [ˈkɒʊ̯ˑnˠɐs] ^ⓘ) is the second-largest city in Lithuania after Vilnius, the fourth-largest city in the Baltic States and an important centre of Lithuanian economic, academic, and cultural life. Kaunas was the largest city and the centre of a county [pl] in the Duchy of Trakai of the Grand Duchy of Lithuania and Trakai Palatinate since 1413. In the Russian Empire, it was the capital of the Kaunas Governorate from 1843 to 1915.

Between 1920 and 1939, when (through military action in the Polish–Lithuanian War) Vilnius was seized and controlled by Poland, Kaunas served as the temporary capital of Lithuania. During the interwar period Kaunas was celebrated for its rich cultural and academic life, fashion, construction of countless Art Deco and Lithuanian National Revival architectural-style buildings as well as popular furniture, interior design of the time, and a widespread café culture. The city's interwar architecture is regarded as among the finest examples of European Art Deco and has received the European Heritage Label. This contributed to Kaunas being designated as the first city in Central and Eastern Europe as a UNESCO City of Design, and also to becoming a World Heritage Site in 2023 as the only European city displaying large-scale urbanization during the interwar period and a range of modernist architecture.

Kaunas was selected as the European Capital of Culture for 2022, together with Esch-sur-Alzette and Novi Sad.

The city is the capital of Kaunas County, and the seat of the Kaunas city municipality and the Kaunas District Municipality. It is also the seat of the Roman Catholic Archdiocese of Kaunas. Kaunas is located at the confluence of the two largest Lithuanian rivers, the Nemunas and the Neris, and is near the Kaunas Reservoir, the largest body of water in the whole of Lithuania.

As defined by Eurostat, the population of Kaunas functional urban area, is estimated at 391,153 (as of^[update] 2021), while according to statistics of Kaunas territorial health insurance fund, there are 447,946 permanent inhabitants (as of 2022) in Kaunas and Kaunas district municipalities combined. Moreover, the tertiary education institutions of Kaunas attract thousands of students annually.

The city's name is of Lithuanian origin and most likely derives from a personal name, however the exact person is unknown and it is believed that he was the ruler of Kaunas Castle. The personal name Kaunas is derived from an adjective kaunus which means "who likes to fight". Other possible meaning of the name of the city of Kaunas is that it is derived from an old adjective which is not in use anymore and which meant "deep", "low", "located in the valley".

Before Lithuania regained independence, the city was generally known in English as Kovno, the traditional Slavicized form of its name. The Polish name is Kowno [ˈkɔvnɔ], and the names in Belarusian include Koўна (Kowna [ˈkou̯nɐ]) and Каўнас (Kawnas [ˈkau̯nɐs]). The Yiddish name is קאָװנע Kovne, and the names in German include Kaunas and Kauen. On Carta Marina from 1539, the city was named Cavm. The city and its elderates also have names in other languages (see Names of Kaunas in other languages and names of Kaunas elderates in other languages).

A 16th-century legend in the Bychowiec Chronicle claims that Kaunas was established by the Romans in ancient times. These Romans were supposedly led by a patrician named Palemon, who had three sons: Barcus, Kunas and Sperus. Palemon fled from Rome because he feared the mad Emperor Nero. Palemon, his sons and other relatives travelled to Lithuania. After Palemon's death, his sons divided his land. Kunas got the land where Kaunas now stands. He built a fortress near the confluence of the Nemunas and Neris rivers and the city that grew up there was named after him. A suburban region in the vicinity is named "Palemonas".