A big lie (German: große Lüge) is a gross distortion or misrepresentation of the truth primarily used as a political propaganda technique. The German expression was first used by Adolf Hitler in his book Mein Kampf (1925) to describe how people could be induced to believe so colossal a lie because they would not believe that someone "could have the impudence to distort the truth so infamously". Hitler claimed that the technique had been used by Jews to blame Germany's loss in World War I on German general Erich Ludendorff, who was a prominent nationalist political leader in the Weimar Republic.

According to historian Jeffrey Herf, the Nazis used the idea of the original big lie to turn sentiment against Jews and justify the Holocaust. Herf maintains that Nazi Germany's chief propagandist Joseph Goebbels and the Nazi Party actually used the big lie technique that they described – and that they used it to turn long-standing antisemitism in Europe into mass murder. Herf further argues that the Nazis' big lie was their depiction of Germany as an innocent, besieged nation striking back at "international Jewry", which the Nazis blamed for starting World War I. Nazi propaganda repeatedly claimed that Jews held outsized and secret power in Britain, Russia, and the United States. It further spread claims that the Jews had begun a war of extermination against Germany, and used these to assert that Germany had a right to annihilate the Jews in self-defense.

In the 21st century, the term has been applied to Donald Trump's and his allies' attempts to overturn the result of the 2020 U.S. presidential election, specifically the false claim that the election was stolen through massive voter and electoral fraud. The scale of the claims resulted in Trump supporters attacking the United States Capitol. Later reports indicate that Trump knew he had genuinely lost the election while promoting the narrative. Scholars^[who?] say that constant repetition across many different forms of media is necessary for the success of the big lie technique, as is a psychological motivation for the public to believe the extreme assertions.

Hitler's definition is given in Chapter 10 of Adolf Hitler's Mein Kampf (part of a single paragraph in both the German original and James Murphy's translation):

But it remained for the Jews, with their unqualified capacity for falsehood, and their fighting comrades, the Marxists, to impute responsibility for the downfall precisely to the man who alone had shown a superhuman will and energy in his effort to prevent the catastrophe which he had foreseen and to save the nation from that hour of complete overthrow and shame. By placing responsibility for the loss of the world war on the shoulders of Ludendorff they took away the weapon of moral right from the only adversary dangerous enough to be likely to succeed in bringing the betrayers of the Fatherland to Justice.

All this was inspired by the principle – which is quite true within itself – that in the big lie there is always a certain force of credibility; because the broad masses of a nation are always more easily corrupted in the deeper strata of their emotional nature than consciously or voluntarily; and thus in the primitive simplicity of their minds they more readily fall victims to the big lie than the small lie, since they themselves often tell small lies in little matters but would be ashamed to resort to large-scale falsehoods.

It would never come into their heads to fabricate colossal untruths, and they would not believe others could have the impudence to distort the truth so infamously. Even though the facts which prove this to be so may be brought clearly to their minds, they will still doubt and waver and will continue to think there may be some other explanation. For the grossly impudent lie always leaves traces behind it, even after it has been nailed down, a fact which is known to all expert liars in this world and to all who conspire together in the art of lying.

— Adolf Hitler, Mein Kampf, vol. I, ch. X