Old Frisian was a West Germanic language spoken between the late 13th century and the end of 16th century. It is the common ancestor of all the modern Frisian languages except for the Insular North Frisian dialects, with which Old Frisian shares a common ancestor called Pre–Old Frisian or Proto-Frisian. Old Frisian was spoken by contemporary Frisians who comprised a loose confederacy along the North Sea coast from around modern-day Bruges in Belgium to the Weser in modern-day northern Germany, dominating maritime trade. The vast majority of the surviving literature comprises legal documents and charters, though some poetry, historiographies, and religious documents are attested as well.

Old Frisian was closely related to and shared common characteristics with the forms of English and Low German spoken during the period. Although earlier scholarship contended that Frisian and English had a closer relationship to each other than to Low German, this is no longer the prevailing view. Old Frisian evolved into Middle Frisian around the turn of the 17th century, being largely pushed out by the emergence of Middle Low German as the language of trade in the North Sea. Scholars have argued that the term "Old Frisian" is somewhat misleading, since Old Frisian was contemporary with other Germanic languages during their "Middle" period, such as Middle English and Middle High German.

Morphologically, Old Frisian generally marked for four cases, three grammatical genders, and two tenses, though more complex grammatical functions could be achieved through periphrastic constructions. Its vocabulary comprised a variety of origins including loanwords from Celtic and Slavic languages. Following the Christianization of the Frisians, Latin loans and calques became increasingly common. Word order in Old Frisian was varied; although its typical constituent word order was subject–object–verb, many different word orders are attested in the surviving texts.

Old Frisian was a West Germanic language, which is a part of the larger Germanic language family. It is classified as an Ingvaeonic language along with Old English and Old Saxon. Old Frisian had several distinct regional forms, each leading to later dialects, which were related. According to Rolf Bremmer, the linguistic phylogeny – that is, the relation of these varieties to each other through linguistic descent – can be described thus:

The periods of the Frisian languages are traditionally divided into Pre–Old Frisian (before 1275), Old Frisian (1275–1550), Middle Frisian (1550–1800), and modern Frisian (1800–present), though these dates have varied among scholars. R. L. Trask, for example, puts the end of the Old Frisian period around 1600, while Han Nijdam [Wikidata] suggests it ends about a hundred years earlier. Some scholars such as Germen de Haan have argued that there is no reason to demarcate them this way and that these periods are more in line with literary periods than linguistic change. Despite its name, Old Frisian was contemporary with Middle Dutch, Middle English, and both Middle High and Middle Low German, though there is some overlap with Old Norse.

According to De Haan, what is referred to as "Old Frisian" should really be called "Middle Frisian" and what is called "Middle Frisian" should be referred to as "Early Modern Frisian". De Haan argues that the current nomenclature is misleading and confusing because it incorrectly suggests that Old Frisian is contemporary with other "Old" Germanic languages such as Old English and Old Saxon. Alistair Campbell expressed similar views, arguing that the Frisian spoken between the 14th and 16th centuries are better described as "Middle Frisian". In some contexts, the term "Old Frisian" may also refer to what is called either "Pre–Old Frisian" or "Proto-Frisian", or both the Pre–Old Frisian and Old Frisian periods collectively. Frederik Hartmann, for example, cites Bremmer's analysis of Pre–Old Frisian sound changes but refers to the language as "Old Frisian". Bremmer argues that the origins of the "Old" terminology are based in clout for this period, stating that the view of those attempting to give it the "Old" appellation hope "its antiquity will add to its prestige" while acknowledging that the argument is functionally "arbitrary". Ultimately, Bremmer sides with the application of "Middle" to this period except for the two Rüstring codices based on vowel quality in unstressed syllables, itself based on agreed-upon criteria going back to the work of Jacob Grimm.

Traditionally, English and the Frisian languages were widely regarded as closer to each other than to any other Germanic language. The German linguist Theodor Siebs is commonly associated with popularizing this affinity and is credited with coining the term "Anglo-Frisian languages" in his 1889 dissertation entitled Zur Geschichte der Englisch-friesischen Sprache ('On the History of the Anglo-Frisian Languages'), though linguists like Henry Sweet articulated the concept as early as 1876. Observations about the close relationship are much older than the 19th century, however; it is likely that Anglo-Saxon missionaries during the 7th and 8th centuries saw the two languages as closely related. Datings proposed for a common ancestor of the Anglo-Frisian languages estimate that it was probably fully formed by the 4th or 5th century, diverging shortly thereafter.

This phylogenetic view of English and Frisian is no longer widely accepted. Linguists, such as Arjen Versloot [fy] and Patrick Stiles, have argued that – while English, Frisian, and Low German are correctly believed to have a common Ingvaeonic ancestor – there is no reason to believe that English and Frisian shared a uniquely close genetic relationship thereafter. Some shared linguistic changes do overlap in ways unique to these languages, often at similar times, but these changes do not match in terms of their relative chronology; in other words, these common changes do not appear to have occurred at the same time or in the same order. Instead, some linguists argue that the Ingvaeonic precursor was likely a broad dialect continuum which saw the dialects which later became English and Frisian develop similarly but not as one language. This continuum was spoken across the continental coast of the North Sea prior to the Migration Period, evolving into distinct languages around turn of the 5th century. The continuum model is sometimes broadened to include Old Low Franconian as well. Under this model, the two language groups did experience a series of changes particular to the area along the North Sea between about 450 and 650, which influenced both languages as well as Dutch, Flemish, and probably northern varieties of Low German.