The Altmark Incident (Norwegian: Altmark-saken) was a naval skirmish of the Second World War between United Kingdom and Nazi Germany, occurring in at that time neutral Norwegian waters on 16 February 1940. The Altmark, a German supply ship, was returning to Germany with 299 British merchant sailors on board, who had been picked up from ships sunk by the heavy cruiser Graf Spee, herself scuttled after the Battle of the River Plate in December 1939. It has the unusual distinction of being the last major boarding action fought by the Royal Navy.

Altmark was attempting to return to Germany with the 299 British merchant seamen that had been taken prisoner. After being intercepted by the destroyer HMS Cossack, captained by Philip Vian, the Altmark sought refuge in the fjord Jøssingfjord, but Cossack followed her inside and forced her to ground. The British then boarded the Altmark and after some hand-to-hand fighting with bayonets, overwhelmed the ship and released the prisoners. The phrase used to the prisoners when taking the ship was "The Navy's Here!"

The Norwegians were angered that their neutrality had been infringed and did not want to be dragged into a European war. There had been Franco-British discussions on occupying Norway and Denmark to prevent them falling into German hands. Before they could do this, Germany invaded Denmark and Norway less than a month later - on 9 April 1940 - as a well-planned tactical move rather than anything to do with this skirmish.