Old High German (OHG; German: Althochdeutsch (Ahdt., Ahd.)) is the earliest stage of the German language, conventionally identified as the period from around 500/750 to 1050. Rather than representing a single supra-regional form of German, Old High German encompasses the numerous West Germanic dialects that had undergone the set of consonantal changes called the Second Sound Shift.

At the start of this period, dialect areas reflected the territories of largely independent tribal kingdoms, but by 788 the conquests of Charlemagne had brought all OHG dialect areas into a single polity. The period also saw the development of a stable linguistic border between German and Gallo-Romance, later French.

Old High German largely preserved the synthetic inflectional system inherited from its ancestral Germanic forms. The eventual disruption of these patterns, which led to the more analytic grammar, are generally considered to mark the transition to Middle High German.

Surviving Old High German texts were all composed in monastic scriptoria, so the overwhelming majority of them are religious in nature or, when secular, belong to the Latinate literary culture of Christianity. The earliest instances, which date to the latter half of the 8th century, are glosses—notes added to margins or between lines that provide translation of the (Latin) text or other aid to the reader.

Old High German is generally dated from around 750 to around 1050. The beginning of this period marks the emergence of the Old High German (OHG) written tradition initially limited to glosses, but by the 9th century, it included substantial translations and original compositions. However, the fact that the defining feature of Old High German, the Second Sound Shift (generally called the High German Consonant Shift in English), may have started as early as the 6th century and is complete by 750, means that some take the 6th century to be the start of the period. Alternatively, terms such as Voralthochdeutsch ("pre-OHG") or vorliterarisches Althochdeutsch ("pre-literary OHG") are sometimes used for the period before 750. Regardless of terminology, all recognize a distinction between a pre-literary period and the start of a continuous tradition of written texts around the middle of the 8th century.

The end of the period is less controversial. The sound changes reflected in spelling during the 11th century led to the remodelling of the entire system of noun and adjective declensions. There is also a hundred-year "dearth of continuous texts" after the death of Notker Labeo in 1022. The mid-11th century is widely accepted as marking the transition to Middle High German.

Old High German encompasses the dialects that had undergone the Second Sound Shift during the 6th century—namely all of the Upper and Central German dialects.

The Franks in the western part of Francia (Neustria and western Austrasia) gradually adopted Gallo-Romance by the beginning of the OHG period, with the linguistic boundary later stabilised approximately along the course of the Meuse and Moselle in the east, and the northern boundary probably a little further south than the current boundary between French and Dutch. North of this line, the Franks retained their language, but it was not affected by the Second Sound Shift, which thus separated the Low Franconian or Old Dutch varieties from the more easterly Franconian dialects which formed part of Old High German.