Fort Istibey () is one of the World War II-era fortifications of the Metaxas Line, on Mount Beles, Central Macedonia along Greece's northern border. It is situated 16 km north of Neo Petritsi village on the mount Belasica (Greek: mount Belles) at an altitude of 1.339 meters. The main mission of the fort was to prevent entrance to the country from Bulgaria. It occupied 13 officers and 350 soldiers. It commander in chief was Ksanthos Pikoulakis. The fort was active until April 1941, when it was forced to surrunder to German troops.