Gustav-Adolf Timotheus Hans Bernd Gisevius (14 July 1904 – 23 February 1974) was a German politician, Gestapo and Abwehr officer and diplomat during the Second World War. He was a member of the Military Resistance, who actively participated in the Oster Conspiracy and the 20 July Plot, being among the few survivors. A covert opponent of the Nazi regime, he also served as a liaison in Zürich between Allen Dulles, station chief for the American OSS, and the German Resistance forces in Germany.

Gisevius was a witness for the defense, during the Nuremberg trials, testifying in favour of Hjalmar Schacht, who was acquitted, and against Hermann Göring, Ernst Kaltenbrunner and Wilhelm Keitel.

Gisevius was born in Arnsberg in the Prussian Province of Westphalia. After law school, he joined the Prussian Interior Ministry in 1933 and was assigned to the newly formed Geheime Staatspolizei, or Gestapo. After joining the Gestapo, he immediately had disagreements with his senior, Rudolf Diels, and was discharged. He continued with police work in the Interior Ministry. When Heinrich Himmler took over Police functions in 1936 in the German Reich, he removed Gisevius from office.

Throughout his time working for the Gestapo, Gisevius described himself as living in constant fear, entering and exiting through the back door, clutching a pistol at his side – all resultant from his misgivings with the terror apparatus to which he was assigned, since according to him, it was like "living in a den of murderers". Gisevius later transferred to the Reich Ministry of the Interior. Although he had no position of power, he maintained connections, notably to Arthur Nebe, that kept him informed of the political background. Gisevius joined the secret opposition to Adolf Hitler, began gathering evidence of Nazi crimes (for use in a later prosecution) and attempted to restrain the increasing power of Himmler and the SS. He maintained links with Hans Oster and Hjalmar Schacht.

When the Second World War started, Gisevius joined the German intelligence service, the Abwehr, which was headed by Admiral Wilhelm Canaris, who was secretly an opponent of Hitler. Canaris had surrounded himself with Wehrmacht officers opposed to Hitler and he welcomed Gisevius into this group. Working from the consulate in Zürich, Hans Gisevius was involved in secret talks with the Vatican. Canaris arranged for appointment of Gisevius as Vice Consul in Switzerland, where Gisevius met with Allen Dulles in 1943 and agreed to serve as a liaison for the German opposition to Hitler, an assembly which counted among its members General Ludwig Beck, Abwehr Chief Canaris, and Mayor Carl Goerdeler of Leipzig. Several members of the conspiratorial circle against Hitler including Gisevius, "all kept homes within easy walking distance of each other." According to Gisevius, the original plot to kill Hitler earlier (namely, before the acquiescence of Great Britain over the Sudetenland) was utterly derailed by Neville Chamberlain whose actions he claims "saved Hitler."

Upon returning to Germany, he was investigated by the Gestapo, but released. In 1944, after the failed 20 July assassination attempt against Hitler, Gisevius first hid at the home of his future wife, the Swiss national Gerda Woog, and fled to Switzerland in 1945, making him one of the few conspirators to survive the war. There, he contacted the Swiss authorities.

Peter Hoffmann's biography of Hitler assassination conspirator Claus von Stauffenberg ("Stauffenberg, A Family History," 1992) indicates that after the failure of Stauffenberg's bomb plot in July 1944, Gisevius went into hiding until 23 January 1945, when he escaped to Switzerland by using a passport that had belonged to Carl Deichmann, a brother-in-law of German Count Helmuth James von Moltke, who was a specialist in international law serving in the legal branch of the Foreign Countries Group of the OKW (Oberkommando der Wehrmacht, "Supreme Command of the Armed Forces"). Through the help of the American Allen Dulles in Berne, Switzerland and of the German Legation (in Berne)'s Georg Federer, the passport was modified and a visa obtained for Gisevius that enabled him to escape to Spain.

Gisevius served as a key witness for the defence at the Nuremberg trials when he was called as a witness by defendants Hjalmar Schacht and Wilhelm Frick. His testimony was crucial in securing the acquittal of Schacht on all counts, but Frick was found guilty and executed. His testimony was also particularly damaging to Hermann Göring, Wilhelm Keitel and Ernst Kaltenbrunner, who were all convicted and sentenced to death.