Agri Decumates was a region of the Roman Empire, covering the Black Forest area between the Main river and the sources of Danube and Rhine rivers, presently in southwestern Germany. It was part of both the province of Raetia and the province of Germania Superior.
The only ancient reference to the name comes from the Roman historian Tacitus, thus all we know of the agri decumates refers to the period c. 70-98, in which latter year Germania was written. In his book Tacitus used the term Agri Decumates or decumates agri (Latin for ten-cantons land, appropriated from Celtic decametos) to denote the area beyond (that is, either to the east or to the north) of the rivers Rhine and Danube. The origin and meaning of the name is disputed. It may refer to an unknown place-name Decuma or Decumum. Latin had an expression for a region obliged to pay tribute to the emperor by handing over a tenth of its produce, a tithe: it was agri decumani, not decumates and characteristic of the Republic, not the early Empire. The currently favoured possibility is that, prior to Roman occupation, the area was inhabited by a tribe which divided the area into ten cantons.
According to Tacitus, the region had originally been populated by the Celtic tribe of the Helvetii but soon Germanic Suebi settled there before migrating, around 9 BCE, to Boiohaemum (modern day Bohemia). After the Suebi left, the area was again inhabited by Gauls. Later on, the region was to become part of the Roman Empire.
The military occupation during the reign of Emperor Vespasian (69-79 CE) laid the foundations for Roman settlement and colonization in the region. Under Emperor Domitian, a network of roads was built covering the whole of the territory. The system of roads enabled the authorities to control and safeguard the area, easing communications between the legions stationed on the river Danube and the Rhine by shortening the Roman defenses in the area. Frontier fortifications were constructed between modern Reinbohl and Pförring.
The region flourished during the next two centuries in spite of periods of unrest: around 185/186, a revolt against Roman authority took place, mainly directed against the presence of the Roman military in Argentorate (modern-day Strasbourg).
The agri decumani were occupied by mixed Gaulish settlers: in the second century, inscriptions attest to the Boii, Mediomatrici and Senones. The first devastating incursion of the Germanic Alamanni tribe occurred in 233. After 259, they began to occupy the region.
The Romans controlled the province until the latter half of the 3rd century, when it was evacuated by the emperor Gallienus (259-260) in the face of invading Alamanni and the secession of much of the Western Roman Empire under the regional emperor Postumus.
The area may have been briefly reoccupied under Emperor Aurelian (270-275), during the Roman military resurgence of the late 3rd century CE. Even if this did occur, the re-establishment of Roman rule was brief. After the death of Emperor Probus (282), the area was finally given up and left to the Alamanni. The territory has since remained inhabited by people of Germanic descent continuously to this day. However, Roman settlements were not immediately abandoned. There is evidence of continuity of Roman life well into the 5th century.
J. G. F. Hind has suggested that the inhabitants of the agri decumates are to be found from the later third to the fifth centuries in the decem pagi— also "ten cantons"— transferred west of the Rhine, to the region between that river and the Saar, between Mainz and Metz.