The national flag of Lithuania (Lietuvos vėliava) consists of a horizontal tricolour of yellow, green, and red. It was adopted on 25 April 1918 during Lithuania's first period of independence (in the 20th century) from 1918 to 1940, which ceased with the occupation first by the Soviet Union, and then by Nazi Germany (1941–1944). During the post-World War II Soviet occupation, from 1945 until 1989, the Soviet Lithuanian flag consisted first of a generic red Soviet flag with the name of the republic, then changed to the red flag with white and green bands at the bottom.
The flag was then re-adopted on 20 March 1989, almost a year before the re-establishment of Lithuania's independence and almost three years before the collapse of the Soviet Union. The last alteration to the current flag occurred in 2004, when the aspect ratio changed from 1:2 to 3:5.
The earliest known flags with a Lithuanian identity were recorded in the 15th-century Banderia Prutenorum, written by Jan Długosz. At the Battle of Grunwald in 1410, two distinct flags were present. The majority of the 40 regiments carried a red banner depicting a mounted knight in pursuit. This flag, known as the Vytis, would eventually be used as the Lithuanian war flag, and again in 2004 as the state flag. The remaining regiments carried a red banner displaying the Columns of Gediminas. Those that bore the Vytis were part of the Grand Ducal Lithuanian army, while those who bore the Columns of Gediminas were from Lithuanian nobility. Until the end of the 18th century, when it was annexed by the Russian Empire, the Grand Duchy of Lithuania used the Vytis as its flag.
The birth of the yellow, green, and red tricolour occurred during a drive by other European republics to change their flags. One example that gave life to the idea of the tricolour was the French blue, white, and red flag adopted after the French Revolution. The only tricolour that existed for Lithuania before the yellow, green, and red flag was a green, white, and red flag used to represent Lithuania Minor.