Greek National Road 51 (, abbreviated as EO51) is a single carriageway road in northeastern Greece. It connects the Greek National Road 2 (Florina - Thessaloniki - Alexandroupoli - Turkey) near Feres with the Bulgarian border near Ormenio. The main towns on the GR-51 are Didymoteicho and Orestiada. The entire GR-51 is part of the European route E85. Its length is about 128 km. It runs from south to north, parallel to the river Evros.