Family name Косаи (Kosai 
, Косаи 
) , across the world, is quite a rare last name. The last name Косаи is habitual in Myanmar, where it is quite a rare surname, Algeria, where it is a rare surname, and Thailand. More prevalently, Косаи is the forename as well as the last name.
| VARIANT | COUNTRY |
|---|---|
| Kosai | Japan, Algeria, Thailand, India, Myanmar |
| Косаи | Russia |