Ludwig August Theodor Beck (German: [ˈluːt.vɪç bɛk] ^ⓘ; 29 June 1880 – 20 July 1944) was a German general who served as Chief of the German General Staff from 1933 to 1938. Beck was one of the main conspirators of the 20 July plot to assassinate Adolf Hitler.

Beck was a staff officer in the Truppenamt of the Reichswehr and became a fellow traveller of the Nazis during the Weimar Republic. Beck was appointed Chief of Staff of the German Army shortly after the Nazi rise to power, supporting Hitler's rearmament of Germany and forceful denunciation of the Treaty of Versailles, although he believed Germany needed more time to prepare for another war.

Beck was increasingly disillusioned with Hitler's aggressive foreign policy, the rising totalitarianism of the Nazi regime, and the influence of the SS over the army in military affairs. Beck became a leader of resistance to Nazism in military circles after retiring in 1938 due to public disagreements with Hitler on foreign policy, and later planned the 20 July plot with Claus von Stauffenberg. Beck was arrested by Friedrich Fromm when the plot failed and executed by one of Fromm's men after a botched suicide attempt.

Ludwig August Theodor Beck was born on 29 June 1880 in Biebrich in Hesse-Nassau (now a borough of Wiesbaden, Hesse) to industrialist Ludwig Georg Ernst Wilhelm Beck and his wife Bertha (née Draudt). He served on the Western Front during the First World War as a staff officer. After the war, he served in various staff and command appointments. In 1931 and 1932, he led the group of army writers, at the Department of the Army (Truppenamt), which published the German Army Operations Manual, Truppenführung. The first section was promulgated in 1933 and the second section in 1934. A modified version is still in use today by the Bundeswehr. He was promoted to Generalleutnant in 1932. Two years later, in 1934, he replaced General Wilhelm Adam as chief of the Truppenamt, the camouflaged General Staff (the Treaty of Versailles explicitly forbade the existence of the General Staff).

In September and October 1930, Beck was a leading defence witness at the trial in Leipzig of three junior Reichswehr officers: Lieutenant Richard Scheringer, Hans Friedrich Wendt and Hanns Ludin. The three men were charged with being members of the Nazi Party since members of the Reichswehr were forbidden from membership of political parties. The three officers admitted their membership and used as their defence the claim that Nazi Party membership should not be forbidden to military personnel. When the three officers were arrested after being caught distributing Nazi literature at their base, Beck, the commanding officer of the 5th Artillery Regiment based in Ulm, to which the three officers belonged, was furious and argued that since the Nazi Party was a force for good, Reichswehr personnel should not be banned from joining the party. At the preliminary hearing, Beck spoke on behalf of the three officers.

At the Leipzig trial of Ludin and Scheringer, Beck testified to the good character of the accused, described the Nazi Party as a positive force in German life and proclaimed his belief that the Reichswehr ban on Nazi Party membership should be rescinded. When Lieutenant Scheringer spoke of a future war in which the Nazi Party and the Reichswehr would fight hand in hand as brothers in a "war of liberation" to abrogate the Treaty of Versailles, Beck supported Scheringer by testifying, "The Reichswehr is told daily that it is an army of leaders. What is a young officer to understand by that?"

Historians such as Sir John Wheeler-Bennett have noted that Beck was deliberately distorting the principle of Hans von Seeckt's Führerarmee ("Army of Leaders"), which trained soldiers to be leaders for when the army would be expanded beyond the limits permitted by the Treaty of Versailles, by seeking to apply it to politics.

In 1933, on witnessing the Nazi Machtergreifung, Beck wrote, "I have wished for years for the political revolution, and now my wishes have come true. It is the first ray of hope since 1918". In July 1934, Beck expressed some alarm at Nazi foreign policy involving Germany in a "premature war" after the failed Nazi putsch in Austria, which led Beck to warn that those in "leading positions" must understand that foreign adventures might then lead to Germany being forced to make a "humiliating retreat", which might bring about the end of the regime. In August 1934, after the death of President Paul von Hindenburg, which led to Adolf Hitler's assumption of the roles of powers of the presidency, most notably the position of commander-in-chief, Beck wrote that Hitler's move created "favourable conditions" for the Reichswehr.