'Jack Knife' was a British attempt to land a raiding party behind the German lines in Norway (May 1940).

The plan called for the delivery of a party of the 5/Scots Guards (a battalion of volunteers able to ski), on detachment to the Independent Companies (precursors of the commandos), in the Sognefjord, where it would be met by Norwegian guides before skiing over the mountains to destroy several segments of the railway between Oslo and Bergen.

The British party sailed in the submarine Truant, but this struck a magnetic mine shortly after leaving Rosyth in Scotland and limped home with severe damage. It had been planned to revive the operation during June, but in late May the British decided that the Norwegian campaign was lost, and called off all attempts to revive the undertaking.