Koi or more specifically nishikigoi (literally "brocaded carp"), are ornamental varieties of domesticated common carp (Cyprinus carpio) that are kept for decorative purposes in outdoor koi ponds or water gardens.

Koi varieties are distinguished by coloration, patterning, and scalation. Some of the major colors are white, black, red, yellow, blue, and cream. The most popular category of koi is the Gosanke, which is made up of the Kohaku, Taisho Sanshoku, and Showa Sanshoku varieties.