Cognomen タダシ (Tadashi 
, タダシ , Tadasi 
) , all over the world, is a common family name. The last name タダシ is habitual in Japan, where it is quite a common surname, Taiwan, where it is quite a rare surname, and Singapore, where it is a rare surname. Measured by absolute frequency, it most frequently occurs in Japan, India, and China. Very seldom, タダシ is the forename as well as the last name.
| VARIANT | COUNTRY |
|---|---|
| Tadashi | Japan, China, Taiwan, India, Brazil |
| タダシ | Japan |
| Tadasi | Japan, China, Philippines, India, Brazil |