'Letterbox' was a British operation by the Special Operations Executive for a two-man Norwegian party to arrange weapons dumps, contact local resistance groups, and begin instruction of resistance fighters in the mountains near Ålesund in German-occupied Norway (15 September/November 1941).

Both members of the party, K. J. Aarsæther and A. Wisløff, returned early in November after making contact with groups around Ålesund.