Ruthenian, or Old Ruthenian (see other names), is a term used for the varieties of Eastern Slavonic spoken in the Grand Duchy of Lithuania and later in the East Slavic territories of the Polish–Lithuanian Commonwealth.

Scholars do not agree whether Ruthenian was a separate language or a Western dialect(s) of Old East Slavic, but it is agreed that Ruthenian has a close genetic relationship with it. Old East Slavic was the colloquial language used in Kievan Rus' (10th – 13th centuries). Ruthenian can be seen as a predecessor of modern Belarusian, Rusyn and Ukrainian. Indeed all these languages, from Old East Slavic to Rusyn, have been labelled as Ruthenian (Ukrainian: Рутенська мова).

In modern texts, the language in question is sometimes called "Old Belarusian" or starabiełaruskaja mova (Belarusian: “Старабеларуская мова”) and "Old Ukrainian" or staroukrajinska mova (Ukrainian: “Староукраїнська мова”). As Ruthenian was always in a kind of diglossic opposition to Church Slavonic, this vernacular language was and still is often called prosta(ja) mova (Cyrillic проста(я) мова, literally "simple language". Contemporary sources only rarely draw any distinction between the dialect of the Grand Duchy of Lithuania and the dialect of the Grand Principality of Vladimir.^{[citation needed]}