The Polish Underground State [a] was a single political and military entity formed by the union of resistance organizations in occupied Poland that were loyal to the Government of the Republic of Poland in exile in London. The first elements of the Underground State were established in the final days of the German and Soviet invasion of Poland, in late September 1939. The Underground State was perceived by supporters as a legal continuation of the pre-war Republic of Poland that waged an armed struggle against the country's occupying powers: Nazi Germany and the Soviet Union. The Underground State encompassed not only military resistance, one of the largest in the world,[b] but also civilian structures, such as justice, education, culture and social services.

In Poland, the resistance movement during World War II was led by the Home Army. The Polish resistance is notable among others for disrupting German supply lines to the Eastern Front, and providing intelligence reports to the British intelligence agencies. It was a part of the Polish Underground State.