Surname Sidorjak (Sidorjak 
, Сидоряк 
) , throughout the world, is a rare family name. The cognomen Sidorjak is characteristic of Eastern Europe, particularly Ukraine, where it is quite a rare surname, Slovakia, where it is a rare surname, and Czechia. Not in comparison with a country's population, it is commonest in Eastern Europe, particularly Ukraine, Russia, and Slovakia.