Operation Tannenberg (German: Unternehmen Tannenberg, Polish: Operacja Tannenberg) was one of the first anti-Polish extermination actions by Nazi Germany in German-occupied Poland from September 1939 to January 1940. The operation was conducted with the use of the Sonderfahndungsbuch Polen, a proscription list of more than 61,000 members of the Second Polish Republic's elite were to be arrested then interned or shot.
Around 20,000 Poles were arrested and killed by the Einsatzgruppen in a number of mass killings during Operation Tannenberg, which was followed by the shooting and gassing of hospital patients and disabled adults as part of the wider Aktion T4 programme.