Llansteffan, also anglicized as Llan- or Lanstephan, is a village and a community situated on the south coast of Carmarthenshire, Wales, lying on the estuary of the River Tywi, 7 miles (11 km) south of Carmarthen.
Llansteffan viewed from Ferryside
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The parish of Llansteffan consists of two distinct villages with separate churches: Llansteffan by the estuary and Llanybri inland on the hilltop. St Ystyffan's church is a grade II* listed building. Between the castle and village sits Plas Llanstephan, Lord Kylsant's former residence, which is also a grade II* listed building 
Llansteffan Castle, built by the Normans in the 12th century and granted to the Marmion family, stands above the village on a promontory commanding the estuary passage. Located between the ferry crossing-points of the Tywi and Tâf rivers, Llansteffan was an important staging post on the Normans' coastal route from Glamorgan via Kidwelly to Pembroke.
- Dylan Thomas had strong family links to Llansteffan. The triangle formed by Llangynog, Llangain and Llansteffan constitutes as Thomas once put it, his "breeding-box valley". His mother's family, the Williamses, lived within this triangle in farms such as Waunfwlchan, Llwyngwyn, Maesgwyn and Penycoed. His mother’s half-sister, Anne, lived in Rose Cottage in the village.
- Osi Rhys Osmond, artist, lived in Llansteffan for 30 years until his death.
- Lieutenant Tomos Stephens, an SAS soldier who was taken prisoner during Operation Bulbasket in World War II, then beaten to death by a German officer.
- "Community population 2011". Retrieved 16 April 2015.
- "Church of St Ystyffan, Llansteffan". British Listed buildings. Retrieved 20 December 2013.
- "Y Plas, Llansteffan". British Listed buildings. Retrieved 20 December 2013.
- "Ward population 2011". Retrieved 16 April 2015.
- Dylan Remembered 1914-34 vol 1 by D N Thomas, Seren 2003, as well as A True Childhood: Dylan’s Peninsularity by D. N. Thomas in Dylan Thomas: A Centenary Celebration ed. H. Ellis, Bloomsbury 2014, which is also online at A True Childhood: Dylan's Peninsularity
- "SAS World War Two hero honoured 75 years after death in France". BBC News. 3 July 2019.