Kaunas Fortress (Lithuanian: Kauno tvirtovė, Russian: Кοвенская крепость, German: Festung Kowno) 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 mi²).

The fortress was battle-tested in 1915 when Germany attacked the Russian Empire, and withstood eleven days of assault before capture. After World War I, the fortress' military importance declined as advances in weaponry rendered it increasingly obsolete. It was used by various civil institutions and as a garrison.

During World War II, parts of the fortress complex were used by the Nazi Germany for detention, interrogation, and execution. About 50,000 people were executed there, including more than 30,000 Jewish victims of the Holocaust. Some sections have since been restored; the Ninth Fort houses a museum and memorial devoted to the Jewish victims of Holocaust mass executions. The complex is the most complete remaining example of a Russian Empire fortress.

The city of Kaunas is located at the confluence of two rivers, the Neman and Neris, which link Lithuania's interior and its capital, Vilnius, to the Baltic Sea. The Baltic peoples had created significant domains by the 1st century, and came into conflict with the Scandinavians and the Slavs; the Teutonic Order began targeting Lithuanian lands at the beginning of the 13th century. Since Lithuania was heavily wooded and its lands were often impassable, its interior was most approachable along its rivers when frozen and during the short dry harvest season in late summer. In response to this vulnerability, defensive structures, including a brick castle in Kaunas, were in place at various points on the Nemunas River by the 14th century. The city was first mentioned in written sources in 1361; it received Magdeburg rights, regulating its autonomy and establishing trade protocols, from Vytautas the Great in 1408. An outpost of the Hanseatic League was created there in 1441. By the end of the 16th century Kaunas had become a major regional trade center, but plagues, fires, and wars adversely affected the country and city during the 17th and 18th centuries. Following the partitions of the Polish–Lithuanian Commonwealth at the end of the 18th century, Lithuania was incorporated into the Russian Empire. Two major 19th century projects contributed to the city's revival. The Augustów Canal, completed in 1832, linked the Neman to the Black Sea, and a rail line linking Saint Petersburg, Warsaw, and Germany via Kaunas was completed in 1862; it was part of a limited network of western Russian railways.

Russia's western borders needed support, and fortresses existed or were being built in Latvia, Ukraine, and Belarus. The concept of building a fortress in Lithuania was discussed without result in 1796, but became a critical concern after the French invasion of Russia in 1812 led by Napoleon. The Grande Armée managed to cross the Nemunas near Kaunas on its drive towards Moscow without major difficulties. An increasingly unified Germany troubled the Empire during the second half of the century. A fortress in Kaunas would present an obstacle to attacks from the west, preventing further incursions towards Riga and Vilnius. In order to control the region, attackers would need to first neutralize Kaunas. Facing this possibility and evaluating the natural advantages of the city, Russian officials decided to construct a fortress there. After several delays, on July 7, 1879 Tsar Alexander II issued an edict ordering its construction.

The first design was overseen by Generals Nikolay Obruchev, Konstantin Zverev, and Ivan Volberg. As originally planned, the fortress encompassed a huge site, consisting of seven forts and nine defensive batteries arranged in concentric loops. The plan included support buildings and infrastructures, such as barracks, new roads, and an ammunition depot. Construction began in 1882; about 4,000 workers were mustered for the project. The principal structures were concentrated in Freda, Panemunė, Aleksotas, and the new section of the city. The project significantly affected the daily life of Kaunas residents, and there were plans to detach the fortress into an independent administrative unit governed by a military board; its commandant wrote that "There is no city of Kaunas, there is only the Fortress of Kaunas."

The first forts were built using bricks reinforced with thick ramparts of earth, which were incorporated into the surrounding relief, making them harder to breach. They were symmetrical, usually having five faces, with positions for infantry and artillery. These forts were built according to the standard Russian brick fort design of the time. Therefore, the first seven forts were very similar; they differed only in the layout of their interiors, their integration into the surrounding relief, and in some construction details. They would also be renovated in slightly different ways. Batteries were built between adjacent forts; these were fortifications containing various types of artillery, located along the fortress' outer lines and usually erected on the hills. The first construction phase was completed in 1887. The fortress was designated first-class in that year, marking its importance and defensive capabilities, and Otto Klem was named its first commandant. At the same time, administrative rules were established to manage the fortress' impact on the city and its surrounding areas; the height of the fortress' civil buildings was restricted.

During 1890 work began on the Eighth fort, known as Linkuva; new construction techniques were introduced, particularly reinforced concrete. The Linkuva fort became the most modern entrenchment, equipped with electricity, sewerage, and casemates for a garrison of 1,000 personnel. At the same time, massive groundworks were laid along with additional defensive structures, effecting the complete enclosure of the city center between the Rivers Nemunas and Neris. By 1890, seven forts had been completed, supporting roads had been constructed, and a railroad bridge over the Nemunas had been adapted for military transport. By now, expenditures on the fortress had amounted to over nine million rubles. The first bricks for a church that would serve the garrison were laid in 1891; it was completed in 1893. The following year construction began on a dedicated narrow gauge railway.