Alien priories were religious establishments in England, such as a monastery or convent, which were under the control of another religious house outside England. Usually the mother-house was located in France.
Alien Priories were small dependencies of foreign religious houses. Specifically, this pertained to the English possessions of French religious houses.
The precedent went back at least as far as 912. Ælfthryth, daughter of Alfred the Great married Baldwin II, Count of Flanders. She received various properties under her father's will, and gave Lewisham Priory with its dependencies, Greenwich and Woolwich, to the abbey of St Peter at Ghent.Edward the Confessor gave the parish church at Deerhurst, and its lands to the monastery of St Denis.
The practice increased after the Norman Conquest. A number of Norman lords had founded abbeys on their lands in France, which in many cases sent monks to England to manage their property. William the Conqueror gave to the Abbey of Saint-Étienne in Caen, the manors of Frampton and Bincombe in Dorset. During the reign of William Rufus, Hamelin de Balun, founded the Benedictine priory of St Mary, at Abergavenny under the jurisdiction of the at Le Mans. The designation "Alien Priory" included any property owned by the French houses, regardless of whether there was an actual priory constructed upon it.
Some priories enjoyed more autonomy than others. A distinction was drawn between those where the prior was appointed and served at the will of the abbot of the motherhouse, and those where the prior was elected by the monks. In the latter case, the land was vested in the priory and could not be sold by the abbot. The priories paid an apport, a nominal fixed sum, annually to the mother-house.