Add to ORKGKM3NeT is a deep‐sea multidisciplinary observatory in the Mediterranean Sea that will provide innovative science opportunities spanning Astroparticle Physics and Earth and Sea Science. This is possible through the synergy created by the use of a common infrastructure allowing for long term continuous operation of a neutrino telescope and marine instrumentation. The present KM3NeT Design Study concludes with this Technical Design Report which develops the ideas put forward in the Conceptual Design Report published in April 2008