Cognomen セイイチ (Seiichi 
, セイイチ , Seiiti 
) , in the world, is a common surname. The family name セイイチ is habitual in Japan, where it is quite a common surname, Taiwan, where it is a rare surname, and Singapore. In absolute terms, it is the most numerous in Eastern Asia, particularly Japan, China, and Taiwan. Much less frequently, セイイチ is the forename as well as the last name.