OST-Arbeiter (German: Ostarbeiter, English: Eastern Workers) was a designation for slave workers gathered from Eastern Europe to do forced labor in Germany during World War II. The Ostarbeiters were mostly from the territory of Reichskommissariat Ukraine (eastern Ukraine). Ukrainians made up the largest portion although many Belarusians, Russians, Poles and Tatars were also present. Estimates put the number of OST-Arbeiters between 3 million and 5.5 million. Some estimates place the number as high as 8.5 million workers.^[who?]

Most were very young, under the age of 16, as those older than 16 were usually conscripted. 30% were as young as 12–14 years of age when they were taken to Germany. By November 1943 the age limit was dropped to 10. 50% of those taken from Ukraine were girls and women.

OST-Arbeiters from Reichskommissariat Ukraine were forced to wear a dark blue and white badge with "OST", the German word for East.

The official German records for the late summer of 1944 listed 7.6 million foreign civilian workers and prisoners of war in the territory of the "Greater German Reich", who for the most part had been brought there for employment by force. Thus, they represent roughly a quarter of all registered workers in the entire economy of the German Reich at that time.