Lleision ap Thomas or Llyson Thomas etc. (died c. 1541) was the last abbot of Neath in West Glamorgan, Wales.He was one of the most influential Cistercians in Wales in the period before the Dissolution of the Monasteries.He is widely rumoured to have settled as a farmer in the upper Swansea valley, raising a family who remain in the locality of the name Leyshon