Kozjak (Macedonian: Козјак (help·info)) is a mountain situated on the border between the Republic of Macedonia and Serbia. It is situated north-east from Kumanovo. The tallest peak of Kozjak is Virovi at 1284 meters. The region is also known as Kozjačija.
During World War II Kozjak was occupied by Draža Mihajlović's Chetniks, and was a ground of severe battles between them and the Macedonian National Liberation Army.
Kozjak is famous for the Prohor Pčinjski monastery in which the first session of the ASNOM was held on 2 August 1944.