Red Ruthenia, also called Red Rus' or Red Russia, is a term used since the Middle Ages for the south-western principalities of Kievan Rus', namely the Principality of Peremyshl and the Principality of Belz. It is closely related to the term Cherven Cities ("Red Cities").

First mentioned by that name in a Polish chronicle of 1321, Red Ruthenia was the portion of Ruthenia incorporated into Poland by Casimir the Great during the 14th century.^{[citation needed]} Following the Mongol invasion of Kievan Rus' in the 13th century, Red Ruthenia was contested by the Grand Duchy of Lithuania (the Gediminids), the Kingdom of Poland (the Piasts), the Kingdom of Hungary and the Kingdom of Galicia–Volhynia. After the Galicia–Volhynia Wars, for about 400 years, most of Red Ruthenia became part of Poland as the Ruthenian Voivodeship.

Nowadays, the region comprises parts of western Ukraine and adjoining parts of south-eastern Poland. It has also sometimes included parts of Lesser Poland, Podolia, Right-bank Ukraine and Volhynia. Centred on Przemyśl and Belz, it has included major cities such as: Chełm, Zamość, Rzeszów, Krosno and Sanok (now all in Poland), as well as Lviv and Ternopil (now in Ukraine).

The first known inhabitants of northern Red Ruthenia were Lendians and White Croats, while subgroups of Rusyns, such as Boykos and Lemkos, lived in the south.

Later Walddeutsche ("Forest Germans"), Jews, Armenians and Poles also made up part of the population. According to Marcin Bielski, although Bolesław I Chrobry settled Germans in the region to defend the borders against Hungary and Kievan Rus' the settlers became farmers. Maciej Stryjkowski described German peasants near Rzeszów, Przemyśl, Sanok, and Jarosław as good farmers. Casimir the Great settled German citizens on the borders of Lesser Poland and Red Ruthenia to join the acquired territory with the rest of his kingdom. In determining the population of late medieval Poland, colonisation and Polish migration to Red Ruthenia, Spiš and Podlachia (whom the Ukrainians called Mazury—poor peasant migrants, chiefly from Mazowsze) should be considered.

During the second half of the 14th century, the Vlachs arrived from the southeastern Carpathians and quickly settled across southern Red Ruthenia. Although during the 15th century the Ruthenians gained a foothold, it was not until the 16th century that the Wallachian population in the Bieszczady Mountains and the Lower Beskids was Ruthenized. From the 14th to the 16th centuries Red Ruthenia underwent rapid urbanization, resulting in over 200 new towns built on the German model (virtually unknown before 1340, when Red Ruthenia was the independent Kingdom of Halych).

A minority of ethnic Poles have lived since the beginning of the second Millennium in northern parts of Red Ruthenia. The exonym "Ruthenians" usually refers to members of the Rusyn and/or Ukrainian ethnicity.

During the early Middle Ages, the region was part of Kievan Rus' and, from 1199, the independent Kingdom of Galicia–Volhynia.^{[citation needed]}