Frankish (reconstructed endonym: *Frankisk), also known as Old Franconian or Old Frankish, was the West Germanic language spoken by the Franks from the 5th to 10th centuries.

Franks under King Chlodio settled in Roman Gaul in the 5th century. One of his successors, named Clovis I, took over the Roman province of Gallia Lugdunensis (in modern day France). Outnumbered by the local populace, the ruling Franks there adapted to its language which was a Proto-Romance dialect. However, many modern French words and place names are still of Frankish origin.

Between the 5th and 10th centuries, Frankish spoken in Northeastern France, present-day Belgium, and the Netherlands is subsequently referred to as Old Dutch, whereas the Frankish varieties spoken in the Rhineland were heavily influenced by Elbe Germanic dialects and the Second Germanic consonant shift and formed part of the modern Central Franconian and Rhine Franconian dialects of German and Luxembourgish.

The Old Frankish language is poorly attested and mostly reconstructed from Frankish loanwords in Old French, and inherited words in Old Dutch, as recorded from the 6th to 12th centuries. A notable exception is the runic Bergakker inscription, which may represent a primary record of 5th-century Frankish, though it is debated whether the inscription is written in Frankish or Old Dutch.

Germanic philology and German studies have their origins in the first half of the 19th century when Romanticism and Romantic thought heavily influenced the lexicon of the linguists and philologists of the time, including pivotal figures such as the Brothers Grimm. As a result, many contemporary linguists tried to incorporate their findings in an already existing historical framework of "stem duchies" and Altstämme (lit. "old tribes", i.e. the six Germanic tribes then thought to have formed the "German nation" in the traditional German nationalism of the elites) resulting in a taxonomy which spoke of "Bavarian", "Saxon", "Frisian", "Thuringian", "Swabian" and "Frankish" dialects. While this nomenclature became generally accepted in traditional Germanic philology, it has also been described as "inherently inaccurate" as these ancient ethnic boundaries (as understood in the 19th century) bore little or limited resemblance to the actual or historical linguistic situation of the Germanic languages. Among other problems, this traditional classification of the continental West Germanic dialects can suggest stronger ties between dialects than is linguistically warranted. The Franconian group is a well known example of this, with East Franconian being much more closely related to Bavarian dialects than it is to Dutch, which is traditionally placed in the Low Franconian sub-grouping and with which it was thought to have had a common, tribal origin.

In a modern linguistic context, the language of the early Franks is variously called "Old Frankish" or "Old Franconian" and refers to the language of the Franks prior to the advent of the High German consonant shift, which took place between 600 and 700 AD. After this consonant shift the Frankish dialect diverges, with the dialects which would become modern Low Franconian not undergoing the consonantal shift, while all others did so to varying degrees. As a result, the distinction between Old Dutch and Old Frankish is largely negligible, with Old Dutch (also called Old Low Franconian) being the term used to differentiate between the affected and non-affected variants following the aforementioned Second Germanic consonant shift.

The Germanic languages are traditionally divided into three groups: West, East and North Germanic. Their exact relation is difficult to determine, and they remained mutually intelligible throughout the Migration Period, rendering some individual varieties difficult to classify.

The language spoken by the Franks was part of the West Germanic language group, which had features from Proto-Germanic in the late Jastorf culture (c. 1st century BC). The West Germanic group is characterized by a number of phonological and morphological innovations not found in North and East Germanic. The West Germanic varieties of the time are generally split into three dialect groups: Ingvaeonic (North Sea Germanic), Istvaeonic (Weser–Rhine Germanic) and Irminonic (Elbe Germanic). While each had its own distinct characteristics, there certainly must have still been a high degree of mutual intelligibility between these dialects. In fact, it is unclear whether the West Germanic continuum of this time period, or indeed Franconian itself, should still be considered a single language or if it should be considered a collection of similar dialects.