Operation Reinhard or Operation Reinhardt (German: Aktion Reinhard or Aktion Reinhardt; also Einsatz Reinhard or Einsatz Reinhardt) was the codename of the secret German plan in World War II to exterminate Polish Jews in the General Government district of German-occupied Poland. This deadliest phase of the Holocaust was marked by the introduction of extermination camps. The operation proceeded from March 1942 to November 1943; about 1.47 million or more Jews were murdered in just 100 days from late July to early November 1942, a rate which is approximately 83% higher than the commonly suggested figure for the kill rate in the Rwandan genocide. In the time frame of July to October 1942, the overall death toll, including all killings of Jews and not just Operation Reinhard, amounted to two million killed in those four months alone. It was the single fastest rate of genocidal killing in history.

During the operation, as many as two million Jews were sent to Bełżec, Sobibór, and Treblinka to be murdered in purpose-built gas chambers. In addition, facilities for mass-murder using Zyklon B were developed at about the same time at the Majdanek concentration camp and at Auschwitz II-Birkenau, near the earlier-established Auschwitz I camp.

After the German–Soviet war began, the Nazis undertook their European-wide "Final Solution to the Jewish Question". In January 1942, during a secret meeting of German leaders chaired by Reinhard Heydrich, Operation Reinhard was drafted; it was soon to become a significant step in the systematic murder of the Jews in occupied Europe, beginning in the General Government district of German-occupied Poland. Within months, three top-secret camps (at Bełżec, Sobibór, and Treblinka) were built to efficiently murder tens of thousands of Jews every day.

These camps differed from Auschwitz and Majdanek, which initially operated as forced-labour camps before facilities for mass death were installed (gas chambers, crematoria etc.) Even after becoming death camps fitted with crematoria, konzentrationslager like Auschwitz remained largely dedicated to forced labor. Unlike "mixed" extermination camps, the extermination camps of Operation Reinhard kept no prisoners, except as needed to advance the camp's industrial-scale murder. Very few Jews escaped death (notably, only two at Bełżec). All other victims were murdered on arrival.

The organizational apparatus behind the new extermination plan had been put to the test already during the "euthanasia" Aktion T4 programme ending in August 1941, during which more than 70,000 Polish and German disabled men, women, and children were murdered. The SS officers responsible for the Aktion T4, including Christian Wirth, Franz Stangl, and Irmfried Eberl, were all given key roles in the implementation of the "Final Solution" in 1942.

The origin of the operation's name is debated by Holocaust researchers. Various German documents spell the name differently, some with "t" after "d" (as in "Aktion Reinhardt"), others without it. Yet another spelling (Einsatz Reinhart) was used in the Höfle Telegram. It is generally believed that Aktion Reinhardt, outlined at the Wannsee Conference on 20 January 1942, was named after Reinhard Heydrich, the coordinator of the so-called Final Solution of the Jewish Question, which entailed the extermination of the Jews living in the European countries occupied by Nazi Germany. Heydrich was attacked by British-trained Czechoslovakian agents on 27 May 1942 and died of his injuries eight days later. The earliest memo spelling out Einsatz Reinhard was relayed two months later.

On 13 October 1941, SS and Police Leader Odilo Globočnik headquartered in Lublin received an oral order from Himmler – anticipating the fall of Moscow – to begin the construction of the first extermination camp at Bełżec in the General Government territory of occupied Poland. The order preceded the Wannsee Conference by three months. The new camp was operational from 17 March 1942, with leadership brought in from Germany under the guise of Organisation Todt (OT).

Globočnik was given control over the entire programme. All the secret orders he received came directly from Himmler and not from SS-Gruppenführer Richard Glücks, head of the greater Nazi concentration camp system, which was run by the SS-Totenkopfverbände and engaged in slave labour for the war effort. Each death camp was managed by between 20 and 35 officers from the Totenkopfverbände (Death's Head Units) sworn to absolute secrecy, and augmented by the Aktion T4 personnel selected by Globočnik. The extermination program was designed by them based on prior experience from the forced euthanasia centres. The bulk of the actual labour at each "final solution" camp was performed by up to 100 mostly Ukrainian Trawniki guards, recruited by SS-Hauptsturmführer Karl Streibel from among the Soviet prisoners of war, and by up to a thousand Sonderkommando prisoners whom the Trawniki guards used to terrorise. The SS called their volunteer guards "Hiwis", an abbreviation of Hilfswillige (lit. "willing to help"). According to the testimony of SS-Oberführer Arpad Wigand during his 1981 war crimes trial in Hamburg, only 25 percent of recruited collaborators could speak German.