Although the exact date of the foundation of the Seiça Cenobium is unknown, the oldest documentary reference known about this monastery, located by the Mondego River, dates from 1162, in which Abbot Martinho is present in the grant of the letter of exemption from episcopal rights given to Crúzios, by Bishop D. Miguel Salomão.
Some years later, in 1175, D. Afonso Henriques issues a letter of donation from the Couto de Barra to D. Pelágio Egas (or Paio Egas), abbot of Santa Maria de Seiça.
At the same time, the Cistercian Order was growing in Portugal, spreading its conventual houses across the territory then regained.
In the reign of D. Sancho I, the establishment of Cistercian communities decreased, with only two monasteries affiliated in Alcobaça, Santa Maria de Maceira Dão in 1188, and Santa Maria de Seiça, donated to the Abbey of Santa Maria de Alcobaça in 1195.
From that date Seiça passed to the Order of Cistercian, hosting a community of white monks.