We report the startling discovery of in situ microdiamonds in a mantle-derived peridotite lens from Bardane, Fjørtoft, western Norway. Diamonds occur within multiphase solid-inclusion assemblages within spinels that are, in turn, inclusions within garnets. Euhedral inclusion-host morphologies of spinel and mineral chemistries all indicate diamond growth from infiltrating fluids at ultrahigh pressure (P) and moderate temperature. These results imply that the Bardane peridotite lens was present within a continental subduction zone at depths of ≥130 km. This paper not only documents the first discovery of in situ microdiamonds within the Caledonian ultrahigh-pressure terrane of western Norway, but also represents the first known global occurrence of subduction-related diamond formation within mantle rocks that have been incorporated into a major continental plate collision zone.