Old Saxony was the homeland of the Saxons who fought the Frankish empire during the Early Middle Ages, until being conquered and converted into a Carolingian stem duchy in the 8th century, the Duchy of Saxony. Contemporary authors such as Bede and the author of the Ravenna Cosmography used the term "Old Saxons" to distinguish them from the Saxons living in Britain, also known as Anglo-Saxons, who they believed had migrated from Old Saxony. Roman sources as far back as the fourth century had described these continental Saxons as northern neighbours of the Franks, who lived near the Lower Rhine. They do not appear to have been politically unified, although they sometimes worked together to fight the Franks. Bede described them as ruled by "satraps".

Old Saxony, like the later duchy of Saxony, included the inland regions later known as Eastphalia, Westphalia and Angria (or Angaria), between the Rhine and Elbe rivers. They possibly also already lived in Nordalbingia, north of the mouth of the Elbe. There is a single uncertain report of a smaller Saxon tribe living in this region in the second century, and this may represent the origin of their name. During their wars with the Frankish empire, the Saxons were described as living north of the Thuringians, as far east as the Elbe. By the time of the Carolingians, most of the North Sea coast between the Rhine and Elbe was inhabited not by the Saxons, but by the medieval Frisians, who were seen as distinct from the Saxons.

Apart from the Frisian coastal area, the Duchy which was formed from this homeland was contained the modern German federal state of Lower Saxony, together with the eastern, Westphalian, part of modern North Rhine-Westphalia, the Nordalbingian part of part of Schleswig-Holstein) and western part of Saxony-Anhalt (Eastphalia), which all lie in northwestern Germany. These old Saxon regions should not be confused with the modern German state of Saxony, which is in eastern Germany, adjoining the northwest border of the Czech Republic.

Ptolemy's Geographia, written in the 2nd century, is sometimes considered to contain the first mentioning of the Saxons. Some copies of this text mention a tribe called Saxones in the area to the north of the lower River Elbe, thought to derive from the word Sax or stone knife.

However, other copies call the same tribe Axones, and it is considered likely that it is a misspelling of the tribe that Tacitus in his Germania called Aviones. These earliest known tribal Saxons inhabited "Northern Albingia", a region bordering the northern bank of the mouth of River Elbe in what is now Western Holstein. As land became scarce, the Saxon population began to expand southward where it absorbed indigenous populations such as Cherusci, Chamavi and Chatti, and remaining portions of the Langobardi (Lombards) and Suebi. This broader domain is called "Old Saxony". The Chauci, according to Tacitus, also lived in the general area later known as Old Saxony and were highly respected among Germanic tribes. He describes them as peaceful, calm, and levelheaded. At some point they may have merged with, or were perhaps synonymous to, the Saxons.

It has been claimed that the Old Saxons were composed of an aristocracy of nobles, a free warrior class of distinction and renown, leading freemen united and controlled by ancient custom of kindred and clan.

Social differences were jealously guarded by social prescription. The death penalty was imposed on any man who married above his rank; the marriage of a man below his station was severely condemned; bastardy was not tolerated; intermarriage between Saxons and other Germans was frowned upon; and strangers were hated. So tenaciously did the Saxons cling to their ancient customary law that clear traces of these social survivals persisted in Saxony down through the Middle Ages.^[how?]

Saxons had been raiding the eastern seaboard of Britain from here during the 3rd and 4th centuries (prompting the construction of maritime defences in eastern Britain called the Saxon Shore) and it is thought that following the collapse of the Roman defences on the Rhine in 407 pressure from population movements in the east forced the Saxons and their neighbouring tribes the Angles and the Jutes to migrate westwards by sea and invade the fertile lowland areas of Britain. The traditional date for this invasion is 449 and is known as the Adventus Saxonum. However, there is little archaeological evidence of any subsequent long-term conflict, and the evidence for large-scale Germanic migration into Britain is equally scant. It is possible that the level of migration was relatively minimal and that the ethnic makeup of the post-Roman population in Britain remained largely unchanged.