Family name יאצק (Jacek 
, יאצק 
) , throughout the world, is a widespread cognomen. The last name יאצק is habitual in Eastern Europe, particularly Poland, where it is a prevalent surname, Hungary, where it is quite a rare surname, and Slovakia. In absolute terms, this name is the most popular in Poland, the United States, and Germany. Much less frequently, יאצק is the forename as well as the last name.