Kaunas Fortress is the remains of a fortress complex in Kaunas, Lithuania. It was constructed and renovated between 1882 and 1915 to protect the Russian Empire's western borders, and was designated a "first-class" fortress in 1887. During World War I, the complex was the largest defensive structure in the entire state, occupying 65 km2 (25 sq mi).

Kaunas 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 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.