A Ruthenian and Ruthene are exonyms of Latin origin, formerly used in Eastern and Central Europe as common ethnonyms for East Slavs, particularly during the late medieval and early modern periods. The Latin term Rutheni was used in medieval sources to describe all Eastern Slavs of the Grand Duchy of Lithuania, as an exonym for people of the former Rus', thus including ancestors of the modern Belarusians, Rusyns and Ukrainians. The use of Ruthenian and related exonyms continued through the early modern period, developing several distinctive meanings, both in terms of their regional scopes and additional religious connotations (such as affiliation with the Ruthenian Greek Catholic Church).

In medieval sources, the Latin term Rutheni was commonly applied to East Slavs in general, thus encompassing all endonyms and their various forms (Belarusian: русіны, romanized: rusiny; Ukrainian: русини, romanized: rusyny). By opting for the use of exonymic terms, authors who wrote in Latin were relieved from the need to be specific in their applications of those terms, and the same quality of Ruthenian exonyms is often recognized in modern, mainly Western authors, particularly those who prefer to use exonyms (foreign in origin) over endonyms.

During the early modern period, the exonym Ruthenian was most frequently applied to the East Slavic population of the Polish–Lithuanian Commonwealth, an area encompassing territories of modern Belarus and Ukraine from the 15th up to the 18th centuries. In the former Austro-Hungarian Monarchy, the same term (German: Ruthenen) was employed up to 1918 as an official exonym for the entire Ukrainian population within the borders of the Monarchy.

Ruteni, a misnomer that was also the name of an extinct and unrelated Celtic tribe in Ancient Gaul, was used in reference to Rus in the Annales Augustani of 1089. An alternative early modern Latinisation, Rucenus (plural: Ruceni) was, according to Boris Unbegaun, derived from Rusyn. Baron Herberstein, describing the land of Russia, inhabited by the Rutheni who call themselves Russi, claimed that the first of the governors who rule Russia is the Grand Duke of Moscow, the second is the Grand Duke of Lithuania and the third is the King of Poland.

According to professor John-Paul Himka from the University of Alberta the word Rutheni did not include the modern Russians, who were known as Moscovitae throughout Western Europe. Vasili III of Russia, who ruled the Grand Duchy of Moscow in the 16th century, was known in European Latin sources as Rhuteni Imperator. Jacques Margeret in his book "Estat de l'empire de Russie, et grande duché de Moscovie" of 1607 said that the name "Muscovites" for the population of Tsardom (Empire) of Russia is an error. During conversations, they called themselves rusaki (which is a colloquial term for Russians) and only the citizens of the capital called themself "Muscovites". Margeret considered that this error is worse than calling all the French "Parisians". Professor David Frick from the Harvard Ukrainian Research Institute has also found in Vilnius the documents from 1655, which demonstrate that Moscovitae were also known in Lithuania as Rutheni. The 16th century Portuguese poet Luís Vaz de Camões in his Os Lusíadas" (Canto III, 11) clearly writes "...Entre este mar e o Tánais vive estranha Gente: Rutenos, Moscos e Livónios, Sármatas outro tempo..." [English translation: "...Between this sea and the Tánais live strange People: Ruthenians, Muscovites and Livonians, Sarmatians in another time..."] differentiating between Ruthenians and Muscovites.

After the partition of Poland, the term Ruthenian referred exclusively to people of the Rusyn- and Ukrainian-speaking areas of the Austro-Hungarian Empire, especially in the Kingdom of Galicia and Lodomeria, Bukovina, and Transcarpathia.

At the request of Mykhailo Levytsky, in 1843, the term Ruthenian became the official name for the Rusyns and Ukrainians within the Austrian Empire. For example, Ivan Franko and Stepan Bandera in their passports were identified as Ruthenians (Polish: Rusini). By 1900, more and more Ruthenians began to call themselves with the self-designated name Ukrainians. With the emergence of Ukrainian nationalism during the mid-19th century, use of "Ruthenian" and cognate terms declined among Ukrainians and fell out of use in Eastern and Central Ukraine. Most people in the western region of Ukraine followed suit later in the 19th century. During the early 20th century, the name Ukrajins'ka mova ("Ukrainian language") became accepted by much of the Ukrainian-speaking literary class in the Austro-Hungarian Kingdom of Galicia and Lodomeria.^{[citation needed]}

Following the dissolution of the Austro-Hungarian Empire in 1918, new states emerged and dissolved; borders changed frequently. After the extremely violent years 1917-23 including the ongoing Russian Civil War, Polish-Soviet War, Lithuanian-Soviet War and Polish-Lithuanian War, the Rusyn and "prosto hovor" (simple speech) speaking people in eastern Austria-Hungary found themselves divided between Czechoslovakia, Poland, and Romania. The labelling of the Ruthenian "simple speech" of the peasants as "Ukrainian" had already begun in Russia and USSR, and was initially associated with the Soviets, who actively sought to spread peasant terms in cities (see Ukrainization) thus evolving the hybrid language Surzhyk.