The scientific case for a neutrino telescope of a cubic kilometre scale is overwhelming. The infra‐structure it requires can easily be shared by a host of other, associated, sciences, making long‐term measurements in the area of oceanography, clima‐tology, geophysics, geotechnics and marine bio‐logical sciences possible. This combination of neu‐trino telescope and multidisciplinary undersea ob‐servatory, KM3NeT, is the subject of this Design Report. It summarises goals for the design and the options for its technical implementation