The Alanmanni or Allemanni, were a German tribe, first mentioned by Dio Cassius during the year, 213 when the Emperor Caracalla fought against them. They apparently dwelt in the basin of the Maine, to the south of the Chatti. This land in southwestern Germany is a broken, highland country, which encourages clannishness and disunity. Alamanni means "All Men" and implies a confederacy of several tribes. According to Asinius Quadratus their name indicates that they were a conglomeration of various tribes.

The confederacy must have been a loose one, however, since several Alamannic kings are recorded as ruling simultaneously, and even as late as the Fifth Century there seems to have been no strong, centralised authority.

There can be little doubt, however, that the ancient Hermunduri formed the preponderating element in the nation, since the area of Germany from which they emerged encompassed the old tribal lands of these peoples. Other tribal groups may include the Juthungi, Bucinobantes, Lentienses, and perhaps the Armalausi. From the 4th century onwards we hear also of the Suebi or Suabi. The Hermunduri had apparently belonged to the Suebi, but it is likely enough that reinforcements from new Suebic tribes had now moved westward. In later times the names Alamanni and Suebi seem to be synonymous.

The tribe was continually engaged in conflicts with the Roman Empire, and in the Third Century, the Alamanni began to trouble the most vulnerable sector of the Roman frontier between the rivers Rhine and Danube, an area finally abandoned by the Empire in 260.

They launched a major invasion of northern Italy in 268, when the Romans were forced to denude much of their German frontier of troops in response to a massive invasion of the Visigoths. In the early summer, the Emperor Gallienus halted their advance in Italy, but then had to deal with the Goths. When the Gothic campaign ended in Roman victory at the Battle of Naissus in September, Gallienus' successor Claudius II Gothicus turned north to deal with the Alamanni, who were swarming over all Italy north of the Po River.
After efforts to secure a peaceful withdrawal failed, Claudius forced the Alamanni to battle at the Battle of Lake Benacus in November. The Alamanni were routed, forced back into Germany, and did not threaten Roman territory for many years afterwards.

Their most famous encounter was at Strassburg, in which Julian defeated them in 357 before he became Emperor, and their king Chonodomarius was taken prisoner. Early in the 5th century the Alamanni appear to have crossed the Rhine and conquered and settled Alsace and a large part of Switzerland. Their kingdom lasted until the year 495, when Clovis I conquered them, from which they formed part of the Frankish dominions. Although some of the Suebi later migrated to Spain and established an independent kingdom there that endured well into the sixth century.

The Alamannic and Swabian dialects are still spoken in German Switzerland, the southern parts of Baden and Alsace, Wurttemberg and a small portion of Bavaria.