Treblinka (Polish: [trɛˈblin.ka]; German: [tʁeˈblɪŋka]) was the second-deadliest extermination camp to be built and operated by Nazi Germany in occupied Poland during World War II. It was in a forest north-east of Warsaw, four kilometres (2+1⁄2 miles) south of the village of Treblinka in what is now the Masovian Voivodeship. The camp operated between 23 July 1942 and 19 October 1943 as part of Operation Reinhard, the deadliest phase of the Final Solution. During this time, it is estimated that between 700,000 and 900,000 Jews were murdered in its gas chambers, along with 2,000 Romani people. More Jews were murdered at Treblinka than at any other Nazi extermination camp apart from Auschwitz-Birkenau.

Managed by the German SS with assistance from Trawniki guards – recruited from among Soviet POWs to serve with the Germans – the camp consisted of two separate units. Treblinka I was a forced-labour camp (Arbeitslager) whose prisoners worked in the gravel pit or irrigation area and in the forest, where they cut wood to fuel the cremation pits. Between 1941 and 1944, more than half of its 20,000 inmates were murdered via shootings, hunger, disease and mistreatment.

The second camp, Treblinka II, was an extermination camp (Vernichtungslager), referred to euphemistically as the SS-Sonderkommando Treblinka by the Nazis. A small number of Jewish men who were not murdered immediately upon arrival became members of its Sonderkommando whose jobs included being forced to bury the victims' bodies in mass graves. These bodies were exhumed in 1943 and cremated on large open-air pyres along with the bodies of new victims. Gassing operations at Treblinka II ended in October 1943 following a revolt by the prisoners in early August. Several Trawniki guards were killed and 200 prisoners escaped from the camp; almost a hundred survived the subsequent pursuit. The camp was dismantled in late 1943. A farmhouse for a watchman was built on the site and the ground ploughed over in an attempt to hide the evidence of genocide.

In the postwar Polish People's Republic, the government bought most of the land where the camp had stood, and built a large stone memorial there between 1959 and 1962. In 1964, Treblinka was declared a national monument of Jewish martyrdom in a ceremony at the site of the former gas chambers. In the same year, the first German trials were held regarding the crimes committed at Treblinka by former SS members. After the end of communism in Poland in 1989, the number of visitors coming to Treblinka from abroad increased. An exhibition centre at the camp opened in 2006. It was later expanded and made into a branch of the Siedlce Regional Museum.

Following the invasion of Poland in 1939, most of the 3.5 million Polish Jews were rounded up and confined to newly established ghettos by the Nazis. The system was intended to isolate the Jews from the outside world in order to facilitate their exploitation and abuse. The supply of food was inadequate, living conditions were cramped and unsanitary, and Jews had no way to earn money. Malnutrition and lack of medicine led to soaring mortality rates. In 1941, the initial victories of the Wehrmacht over the Soviet Union inspired plans for the German colonisation of occupied Poland, including all territory within the new district of General Government. At the Wannsee Conference held near Berlin on 20 January 1942, new plans were outlined for the genocide of Jews, known as the "Final Solution" to the Jewish Question. The extermination programme was codenamed Operation Reinhard. and was separate from the Einsatzgruppen mass-murder operations in Eastern Europe, in which half a million Jews had already been murdered.

Treblinka was one of three secret extermination camps set up for Operation Reinhard; the other two were Bełżec and Sobibór. All three were equipped with gas chambers disguised as shower rooms, for the murder of entire transports of people. The method was established following a pilot project of mobile extermination conducted at Soldau and at Chełmno extermination camp that began operating in 1941 and used gas vans. Chełmno (German: Kulmhof) was a testing ground for the establishment of faster methods of murdering and incinerating bodies. It was not a part of Reinhard, which was marked by the construction of stationary facilities for mass murder. Treblinka was the third extermination camp of Operation Reinhard to be built, following Bełżec and Sobibór, and incorporated lessons learned from their construction. Alongside the Reinhard camps, mass-murder facilities using Zyklon B were developed at the Majdanek concentration camp in March 1942, and at Auschwitz II-Birkenau between March and June.

Nazi plans to murder Polish Jews from across the General Government during Aktion Reinhard were overseen in occupied Poland by Odilo Globocnik, a deputy of Heinrich Himmler, head of the SS, in Berlin. The Operation Reinhard camps reported directly to Himmler. The staff of Operation Reinhard, most of whom had been involved in the Action T4 "involuntary euthanasia" programme, used T4 as a framework for the construction of new facilities. Most of the Jews who were murdered in the Reinhard camps came from ghettos. The Operation Reinhard camps reported directly to Himmler, and not to the concentration camps inspector Richard Glücks.

The two parallel camps of Treblinka were built 80 km (50 mi) northeast of Warsaw. Before World War II, it was the location of a gravel mining enterprise for the production of concrete, connected to most of the major cities in central Poland by the Małkinia–Sokołów Podlaski railway junction and the Treblinka village station. The mine was owned and operated by the Polish industrialist Marian Łopuszyński, who added the new 6 km (3.7 mi) railway track to the existing line. When the German SS took over Treblinka I, the quarry was already equipped with heavy machinery that was ready to use. Treblinka was well-connected but isolated enough, halfway between some of the largest Jewish ghettos in Nazi-occupied Europe, including the Warsaw Ghetto and the Białystok Ghetto, the capital of the newly formed Bialystok District. The Warsaw Ghetto had 500,000 Jewish inmates, and the Białystok Ghetto had about 60,000.