Old Church Slavonic (OCS) or Old Slavonic (/sləˈvɒnɪk, slæˈvɒn-/ slə-VON-ik, slav-ON-) is the first Slavic literary language and the oldest extant written Slavonic language attested in literary sources. It belongs to the South Slavic subgroup of the Balto-Slavic branch of the Indo-European language family and remains the liturgical language of many Christian Orthodox churches.

Historians credit the 9th-century Byzantine missionaries Saints Cyril and Methodius with standardizing the language and undertaking the task of translating the Gospels and necessary liturgical books into it as part of the Christianization of the Slavs. It is thought to have been based primarily on the dialect of the 9th-century Byzantine Slavs living in the Province of Thessalonica (in present-day Greece).

Old Church Slavonic played an important role in the history of the Slavic languages and served as a basis and model for later Church Slavonic traditions. Some Eastern Orthodox and Eastern Catholic churches use these Church Slavonic recensions as a liturgical language to this day.

As the oldest attested Slavic language, Old Church Slavonic provides important evidence for the features of Proto-Slavic, the reconstructed common ancestor of all Slavic languages.

The name of the language in Old Church Slavonic texts was simply Slavic (словѣ́ньскъ ѩꙁꙑ́къ, slověnĭskŭ językŭ), derived from the word for Slavs (словѣ́нє, slověne), the self-designation of the compilers of the texts. This name is preserved in the modern native names of the Slovak and Slovene languages. The terms Slavic and Slavonic are interchangeable in English. The language is sometimes called Old Slavic, which may be confused with the distinct Proto-Slavic language. Bulgarian, Croatian, Macedonian, Serbian, Slovene and Slovak linguists have claimed Old Church Slavonic; thus OCS has also been variously called Old Bulgarian, Old Macedonian, Old Slovenian, Old Croatian, or Old Serbian, or even Old Slovak. The commonly accepted terms in modern English-language Slavic studies are Old Church Slavonic and Old Church Slavic.

The term Old Bulgarian (Bulgarian: старобългарски, German: Altbulgarisch) is the designation used by most Bulgarian-language writers. It was used in numerous 19th-century sources, e.g. by August Schleicher, Martin Hattala, Leopold Geitler and August Leskien, who noted similarities between the first literary Slavic works and the modern Bulgarian language. For similar reasons, Russian linguist Aleksandr Vostokov used the term Slav-Bulgarian. The term is still used by some writers but nowadays normally avoided in favor of Old Church Slavonic.

The term Old Macedonian is occasionally used by Western scholars in a regional context. According to Slavist Henrik Birnbaum, the term was introduced mostly by Macedonian scholars and it is anachronistic because there was no separate Macedonian language, distinguished from early Bulgarian, in the 9th century.

The obsolete term Old Slovenian was used by early 19th-century scholars who conjectured that the language was based on the dialect of Pannonia.