Cognomen 西根 (西根 
, Seaghan 
, Siegen 
) , across the world, is quite a common family name. The last name 西根 is habitual in Japan, where it is quite a rare surname, Switzerland, where it is a rare surname, and Germany, where it is a very rare surname. In absolute terms, it is the most numerous in Japan, China, and Germany. Very seldom, 西根 is the forename as well as the last name.