The Jireček Line is an imaginary line that divided in the ancient Balkans, until the 4th century, the influences of the Latin (in North) and Greek (in South) languages. It goes from near the city of Laçi in current Albania to Serdica (now Sofia, in Bulgaria) and then following the Balkan Mountains to the Black Sea.

The placing of the line is based on archaeological findings: most of the inscriptions found to the north of it were written in Latin, while most of the inscriptions found to the south were in Greek.

This line is important in the establishing of the place where the Romanian and Aromanian people were formed (see Origin of Romanians), since it is considered unlikely that a Latin people formed on the south of it.

It was originally used by the Czech historian Konstantin Jireček in 1911 in a history of the Slavic people.