The Defence of the Polish Post Office in Gdańsk (Danzig) was one of the first acts of World War II in Europe, as part of the September Campaign. On 1 September 1939 the Invasion of Poland was initiated by Germany when the battleship Schleswig-Holstein opened fire on the Polish-controlled harbor of Danzig, around 04:45. Danzig paramilitaries and police, supported by Germany, immediately joined the offensive to take full control of the city, by capturing the Polish post office. Polish personnel defended the building for some 15 hours against assaults by the SS Heimwehr Danzig (SS Danzig Home Defence), local SA formations and special units of Danzig police. All but four of the defenders, who were able to escape from the building during the surrender, were sentenced to death by a German court martial as illegal combatants on 5 October 1939, and executed (the judgement was later acknowledged as judicial murder).

The Polish Post Office (Poczta Polska) in the Free City of Danzig was created in 1920 under the Treaty of Versailles, and its buildings were considered extraterritorial Polish property. The Polish Post Office in Danzig comprised several buildings.

As tensions between Poland and Germany grew, in April 1939 the Polish High Command detached combat engineer and Army Reserve Sublieutenant (or 2LT) Konrad Guderski (1900–1939) to the Baltic Sea coast. With Alfons Flisykowski and others, he helped organise the official and volunteer security staff at the Polish Post Office in Danzig, and prepared them for possible hostilities. In addition to training the staff, he prepared the defences in and around the building: nearby trees were removed and the entrance was fortified. In mid-August, ten additional employees were sent to the post office from Polish Post offices in Gdynia and Bydgoszcz (mostly reserve non-commissioned officers).

In the Polish Post Office complex on 1 September 1939 there were 56 people: Guderski, 42 local Polish employees, ten employees from Gdynia and Bydgoszcz, and the building caretaker with his wife and ten-year-old daughter, Erwina, who lived in the complex. The Polish employees had a cache of weapons, consisting of three Browning wz.1928 light machine guns, 40 other firearms and three chests of hand grenades. The Polish defence plan assigned the defenders the role of keeping Germans from the main building for six hours, when a relief force from Armia Pomorze was supposed to secure the area.

The German attack plan, devised in July 1939, determined that the main building and its defenders would be stormed from two directions. A diversionary attack was to be carried out at the front entrance, while the main force would break through the wall from the neighbouring Work Office and attack from the side. Danzig Police also drew up plans for attacking the post office.

At 04:00 the Germans cut the phone and electricity lines to the building. At 04:45, just as the German battleship Schleswig-Holstein started shelling the nearby Polish Army military outpost at Westerplatte, the Danzig police began their assault on the building under the command of Polizeioberst Willi Bethke. They were soon reinforced by local SA formations and the SS units SS Wachsturmbann "E" and SS Heimwehr Danzig, supported by three police ADGZ heavy armoured cars. Albert Forster, head of the local Nazi party, arrived in one of the vehicles to watch the event. Journalists from local newspapers, Reichssender Danzig (the state radio station), and the newsreel company Ufa-Tonwache also came to cover the battle.

The first German attack, from the front, was repelled, although some Germans managed to break through the entrance and briefly enter the building (at the cost of two killed and seven wounded attackers, including one group leader). The second attack, from the work office, was also repelled. The commander of the Polish defence, Konrad Guderski, died during the second attack from the blast of his own grenade, which stopped the Germans who had broken through the wall. With the Poles putting up more determined resistance than expected, Bethke suggested that his forces blow up the building with high explosives. Forster vetoed his proposal.

At 11:00 German units were reinforced by the Wehrmacht with two 75 mm artillery pieces and a 105 mm howitzer, but the renewed assault, even with the artillery support, was again repulsed. Mortar support was requested from the German forces at Westerplatte, but its inaccurate fire posed a greater threat to the attackers and it soon ceased action. At 15:00, the Germans declared a two-hour ceasefire and demanded that the Polish forces surrender, which they refused. In the meantime, German sappers dug under the walls of the building and prepared a 600 kg explosive device. At 17:00, the bomb was set off, collapsing part of the wall, and German forces under the cover of three artillery pieces attacked again, this time capturing most of the building except the basement.