The Night of the Long Knives (German: Nacht der langen Messer ), or the Röhm purge, also called Operation Hummingbird (German: Unternehmen Kolibri), was a purge that took place in Nazi Germany from June 30 to July 2, 1934. Chancellor Adolf Hitler, urged on by Hermann Göring and Heinrich Himmler, ordered a series of political extrajudicial executions intended to consolidate his power and alleviate the concerns of the German military about the role of Ernst Röhm and the Sturmabteilung (SA), the Nazis' paramilitary organization, known colloquially as "Brownshirts". Nazi propaganda presented the murders as a preventive measure against an alleged imminent coup by the SA under Röhm – the so-called Röhm Putsch.

The primary instruments of Hitler's action, which carried out most of the killings, were the Schutzstaffel (SS) paramilitary force under Himmler and its Security Service (SD), and Gestapo (secret police) under Reinhard Heydrich. Göring's personal police battalion also took part in the killings. Many of those killed in the purge were leaders of the SA, the best-known being Röhm himself, the SA's chief of staff and one of Hitler's longtime supporters and allies. Leading members of the leftist-leaning Strasserist faction of the Nazi Party, including its figurehead, Gregor Strasser, were also killed, as were establishment conservatives and anti-Nazis, such as former Chancellor Kurt von Schleicher and Bavarian politician Gustav Ritter von Kahr, who had suppressed Hitler's Munich Beer Hall Putsch in 1923. The murders of SA leaders were also intended to improve the image of the Hitler government with a German public that was increasingly critical of thuggish SA tactics.

Hitler saw the independence of the SA and the penchant of its members for street violence as a direct threat to his newly gained political power. He also wanted to appease leaders of the Reichswehr, the German military, who feared and despised the SA as a potential rival, in particular because of Röhm's ambition to merge the army and the SA under his own leadership. Additionally, Hitler was uncomfortable with Röhm's outspoken support for a "second revolution" to redistribute wealth. In Röhm's view, President Hindenburg's appointment of Hitler as chancellor on January 30, 1933, had brought the Nazi Party to power, but had left unfulfilled the party's larger goals. Finally, Hitler used the purge to attack or eliminate German critics of his new regime, especially those loyal to Vice-Chancellor Franz von Papen, as well as to settle scores with old enemies.At least 85 people died during the purge, although the final death toll may have been in the hundreds, with high estimates running from 700 to 1,000. More than a thousand perceived opponents were arrested. The purge strengthened and consolidated the support of the military for Hitler. It also provided a legal grounding for the Nazis, as the German courts and cabinet quickly swept aside centuries of legal prohibition against extrajudicial killings to demonstrate their loyalty to the regime. The Night of the Long Knives was a turning point for the German government. It established Hitler as the supreme administrator of justice of the German people, as he put it in his July 13 speech to the Reichstag.

Before its execution, its planners sometimes referred to the purge as Hummingbird (German: Kolibri), the codeword used to send the execution squads into action on the day of the purge. Before the purge, the phrase "Night of the Long Knives" in German referred to acts of vengeance.