Zyklon B (German: [tsyˈklo:n ˈbe:] (About this soundlisten); translated Cyclone B) was the trade name of a cyanide-based pesticide invented in Germany in the early 1920s. It consisted of hydrogen cyanide (prussic acid), as well as a cautionary eye irritant and one of several adsorbents such as diatomaceous earth. The product is infamous for its use by Nazi Germany during the Holocaust to murder approximately one million people in gas chambers installed at Auschwitz-Birkenau, Majdanek, and other extermination camps.

Hydrogen cyanide, a poisonous gas that interferes with cellular respiration, was first used as a pesticide in California in the 1880s. Research at Degesch of Germany led to the development of Zyklon (later known as Zyklon A), a pesticide which released hydrogen cyanide upon exposure to water and heat. It was banned after a similar product was used by Germany as a chemical weapon in World War I. In 1922, Degesch was purchased by Degussa, where a team of chemists that included Walter Heerdt [de] and Bruno Tesch developed a method of packaging hydrogen cyanide in sealed canisters along with a cautionary eye irritant and one of several adsorbents such as diatomaceous earth. The new product was also named Zyklon, but it became known as Zyklon B to distinguish it from the earlier version. Uses included delousing clothing and disinfesting ships, warehouses, and trains.

In early 1942, Zyklon B emerged as the preferred killing tool of Nazi Germany for use in extermination camps during the Holocaust. Around a million people were killed using this method, mostly at Auschwitz. Tesch was executed in 1946 for knowingly selling the product to the SS for use on humans. Hydrogen cyanide is now rarely used as a pesticide, but still has industrial applications. Firms in several countries continue to produce Zyklon B under alternative brand names, including Detia-Degesch, the successor to Degesch, who renamed the product Cyanosil in 1974.