The W21 was a hydrogen bomb design for the US military.
W21 may also refer to:
British NVC community W21, a scrub community in the British National Vegetation Classification system Mercedes-Benz W21, a German sedan Samsung W21 5G, a smartphone Wanderer W21, a German sedan Wilderness 21, an American sailboat The AIATSIS designation for the Tharrkari language The Wenninger index number for a Great dodecahedron