The Kostur dialect (Macedonian: Костурски дијалект, Kosturski dijalekt), is a member of the Southwestern subgroup of the Southern group of dialects of the Macedonian language. This dialect is mainly spoken in and around the town of Kastoria, known locally in Macedonian as Kostur, and in the surrounding Korešta region, (Macedonian: Корешта; in the Kostur dialect: Korèshcha/Корèшча) which encompasses most of the area to the northwest of the town. The Kostur dialect is also partially spoken in Albania, most notably in Bilisht and the village of Vërnik (Vrabnik). The dialect is partially preserved among the descendants of Muslim immigrants from the region of Kastoria in Turkey.
The Kostur dialect shares strong similarities with the Nestram-Kostenar dialect and the Korča dialect. Linguist Stoyko Stoykov regards the Nestram dialect as a subgroup of the Kostur dialect, part of Bulgarian language.
The dialect is commonly viewed as one of the most divergent forms of the Macedonian dialect continuum. Today it is primarily restricted to oral communication among native speakers; however, in the past the dialect was frequently used in its written form. As late as the Greek Civil War the dialect was being used in newspapers and other print. The Nova Makedonka (Macedonian: Нова Македонка, New Macedonian Woman) newspaper published in the period 1948–1949, was published both in the Kostur dialect and in Greek. The Edinstvo newspaper published from 1947–1949 also solely made use of the Kostur dialect.
In 2011 a memoir book in the Kostur dialect using a Bulgarian orthography was published.