The Altmark was a German tanker / unarmed supply vessel, best known for her support of the Admiral Graf Spee and later involvement in the Altmark Incident.

The vessel was 141 meters long, the tonnage was 7.021 GRT. She was build in Hamburg in 1938 by Blohm and Voss.

The Altmark was assigned to the Admiral Graf Spee to support the battleship during her raids in the South Atlantic. Seamen rescued from the ships sunk were transferred to the Altmark. When the Admiral Graf Spee was scuttled in Montevideo the Altmark returned back to Germany, sailing north of Great Britain and along the Norwegian coast.

She entered Norwegian waters in February 1940 with 299 British merchant seamen as prisoners. On February 16 1940 the Altmark was boarded by sailors from the British destroyer HMS Cossack and the merchant sailors set free.

The boarding was a breach of Norwegian neutrality and would be pivotal for the Germans in their decision to attack Norway and Denmark in April 1940.