Flak towers (German: Flaktürme) were large, above-ground, anti-aircraft gun blockhouse towers constructed by Nazi Germany. "Flak" is short for anti-aircraft gun in German: Flugabwehrkanone. There were eight flak tower complexes in the cities of Berlin (three), Hamburg (two), and Vienna (three) from 1940. Other cities that used flak towers included Stuttgart and Frankfurt. Smaller single-purpose flak towers were built at outlying German strongpoints, such as at Angers in France and Heligoland in Germany.

The towers were operated by the Luftwaffe to defend against Allied strategic air raids against these cities during World War II. They also served as air-raid shelters for tens of thousands of local civilians.

After the RAF raid on Berlin in 1940, Adolf Hitler ordered the construction of three massive flak towers to defend the capital from air attack. Each tower had a radar installation with a radar dish which could be retracted behind a thick concrete and steel dome for protection.

Hitler was interested in the design of the towers, and even made some sketches. They were constructed in six months. The priority of the project was such that the German national rail schedule was altered to facilitate the shipment of concrete, steel and timber to the construction sites.

With concrete walls up to 3.5 m (11 ft) thick, their designers considered the towers to be invulnerable to attack by the standard ordnance carried by RAF heavy bombers at the time of their construction.

The towers were able to sustain a rate of fire of 8,000 rounds per minute from their multi-level guns (albeit mostly smaller-caliber shells, such as the 2cm FlaK 30), with a range of up to 14 km (8.7 mi) in a 360-degree field of fire. However, only the 128 mm (5.0 in) FlaK 40 guns had effective range to defend against the RAF and USAAF heavy bombers. The three flak towers around the outskirts of Berlin created a triangle of anti-aircraft fire that covered the centre of Berlin.

The flak towers had also been designed with the idea of using the above-ground bunkers as a civilian shelter, with room for 10,000 civilians and a hospital ward inside. During the Battle of Berlin, occupants formed their own communities, with up to 30,000 Berliners taking refuge in one tower during the battle. These towers, much like the keeps of medieval castles, were some of the safest places in a fought-over city and so the flak towers were some of the last places to surrender to the Red Army, eventually being forced to capitulate as supplies dwindled.

The Soviets, in their assault on Berlin, found it difficult to inflict significant damage on the flak towers, even with some of the largest Soviet guns, such as the 203 mm M1931 howitzers.