Russian is an East Slavic language belonging to the Balto-Slavic branch of the Indo-European language family. It is one of the four extant East Slavic languages, and is the native language of the Russians. It was the de facto and de jure official language of the former Soviet Union. Russian has remained an official language of the Russian Federation, Belarus, Kazakhstan, Kyrgyzstan, and Tajikistan, and is still commonly used as a lingua franca in Ukraine, Moldova, the Caucasus, Central Asia, and to a lesser extent in the Baltic states and Israel.

Russian has over 253 million total speakers worldwide. It is the most spoken native language in Europe, the most spoken Slavic language, and the most geographically widespread language of Eurasia. It is the world's seventh-most spoken language by number of native speakers, and the world's ninth-most spoken language by total number of speakers. Russian is one of two official languages aboard the International Space Station, one of the six official languages of the United Nations, as well as the fourth most widely used language on the Internet.

Russian is written using the Russian alphabet of the Cyrillic script; it distinguishes between consonant phonemes with palatal secondary articulation and those without—the so-called "soft" and "hard" sounds. Almost every consonant has hard-soft counterparts, and the distinction is a prominent feature of the language, which is usually shown in writing not by a change of the consonant but rather by changing the following vowel. Another important aspect is the reduction of unstressed vowels. Stress, which is often unpredictable, is not normally indicated orthographically, though an optional acute accent may be used to mark stress – such as to distinguish between homographic words (e.g. замо́к [zamók, 'lock'] and за́мок [zámok, 'castle']), or to indicate the proper pronunciation of uncommon words or proper nouns.

Russian is an East Slavic language of the wider Indo-European family. Old East Slavic (or Old Russian) is the parent language of the modern Russian, Belarusian and Ukrainian languages. In many places in eastern and southern Ukraine and throughout Belarus, these languages are spoken interchangeably, and in certain areas traditional bilingualism resulted in language mixtures such as surzhyk in eastern Ukraine and trasianka in Belarus. The Novgorod dialect, a historical variety of Russian with unique northwestern dialectal features, is sometimes considered to have played a significant role in the formation of modern Russian. Also, Russian has notable lexical similarities with Bulgarian due to a common Church Slavonic influence on both languages, but because of later developments in the 19th and 20th centuries, modern Bulgarian grammar differs markedly from Russian.

Over the course of centuries, the vocabulary and literary style of Russian have also been influenced by Western and Central European languages such as Greek, Latin, Polish, Dutch, German, French, Italian, and English, and to a lesser extent the languages to the south and the east: Uralic, Turkic, Persian, Arabic, and Hebrew.

According to the Defense Language Institute in Monterey, California, Russian is classified as a level III language in terms of learning difficulty for native English speakers, requiring approximately 1,100 hours of immersion instruction to achieve intermediate fluency.

Feudal divisions and conflicts created obstacles between the Russian principalities before and especially during Mongol rule. This strengthened dialectal differences, and for a while, prevented the emergence of a standardized national language. The formation of the unified and centralized Russian state in the 15th and 16th centuries, and the gradual re-emergence of a common political, economic, and cultural space created the need for a common standard language. The initial impulse for standardization came from the government bureaucracy for the lack of a reliable tool of communication in administrative, legal, and judicial affairs became an obvious practical problem. The earliest attempts at standardizing Russian were made based on the so-called Moscow official or chancery language, during the 15th to 17th centuries. Since then, the trend of language policy in Russia has been standardization in both the restricted sense of reducing dialectical barriers between ethnic Russians, and the broader sense of expanding the use of Russian alongside or in favour of other languages.

The current standard form of Russian is generally regarded as the modern Russian literary language (современный русский литературный язык, sovremenny russky literaturny yazyk), or Contemporary Standard Russian. It arose at the beginning of the 18th century with the modernization reforms of the Russian state under the rule of Peter the Great and developed from the Moscow (Middle or Central Russian) dialect substratum under some influence of the Russian chancery language. The Moscow dialect had a northern dialectal base, but after Moscow became the center of a unified state, the attraction of southern dialectal speakers led to the emergence of a transitional dialect group.