Cognomen Тадаши (Tadashi 
, Tadachi 
, Тадаши 
) , in all the world, is quite a common surname. The last name Тадаши is habitual in Japan, where it is quite a rare surname, Taiwan, and Singapore, where it is a rare surname. In absolute terms, it is commonest in Eastern Asia, particularly Japan, China, and Taiwan. Likewise, Тадаши is the forename as well as the last name.
| VARIANT | COUNTRY |
|---|---|
| Tadashi | Japan, China, Taiwan, United States, Brazil |
| Tadachi | Japan, Philippines, United States, France, Brazil |
| Тадаши | Russia |
| Tadashy | Brazil, United States |