Nippon Kaiji Kyokai (, Ippan-zaidan-hjin Nippon Kaiji Kykai?) is a ship classification society. It is also known by the brand name "ClassNK" or often in the industry as just "NK". It is a not for profit society dedicated to ensuring the safety of life and property at sea, and the prevention of pollution of the marine environment.
The principal work of the Society's expert technical staff is to undertake surveys to ensure that the rules which it has developed, are applied to both newly built and existing ships in order to ensure the safety of these vessels. The rules cover not only hull structures, but also safety equipment, cargo handling gear, engines, machinery, and electrical and electronic systems among others.
By the end of December 2007, the Society had 6793 ships totaling 152.22 million gross tons under class. This figure represents approximately 20 percent of the world merchant fleet currently under class. Although based in Japan, ClassNK has worldwide representation through a network of exclusive surveyor offices. ClassNK's surveyors work in shipbuilding and repair yards and at ports across the world, wherever they may be called upon to examine the condition of a ship so that all of the Society's services are available worldwide. On November 15, 1999, Nippon Kaiji Kyokai celebrated the centenary of its foundation. On 28 May 2012, ClassNK officially announced that its register had surged past the 200 million gross ton mark becoming the world's first class society in history to have more than 200 million gross tons on its register.