The Rafeiro do Alentejo is believed to descend from molosser dogs from the Middle East. On account of their size and courage, they were used by tribes whose livelihood depended on livestock husbandry, thus performing a crucial role for this type of community.
With the beginning of the transhumance in Portugal, it was noticed that the migrating herds and shepherds were exposed to many dangers during their long journeys. Soon their route to the mountains in the summer and back to the plains in the winter, flocks were always accompanied by large dogs, which lead to the dogs being spread along the route from region to region. As the population settled down, the dogs eventually remained in the southern prairies, the Alentejo plains, where they adapted and began to be used to guard, not only the herds but the big rural estates as well.
The breed is likely to be descended from the huge dogs of the Tibetan highlands. The Alentejo Mastiff was recognized by the United Kennel Club in 2006.