Plas Llanstephan is a mansion in the county of Carmarthenshire, Wales. To the south-east of the house sits Llansteffan Castle and, to the north, Llansteffan village. Plas Llanstephan was built in the second half of the 16th century by the Lloyd family. The Lloyd family inhabited Plas until 1767, when the Meares family, of Eastington acquired it; from them it passed to the family of Morris who were bankers in Carmarthen.
For parts of the 19th century, the mansion was leased out to Sir John James Hamilton, and Sir John Williams (1840–1926). It was sold in 1920 to Sir Owen Cosby Philipps (1863–1937), politician and shipping magnate. He was made Lord Kylsant in 1923. The house passed to his wife, Lady Kylsant, on his death. On the death of Lady Kylsant in December 1952 it was inherited by her eldest daughter Nesta Donne Fisher-Hoch. Her husband held the position of High Sheriff of Carmarthenshire during the 1960s.
The grounds at Plas include one of the largest walled gardens in Wales. The stable block is dated 1788 and is thought to have been designed by John Nash. The house was most recently sold in 2000: at this time it was in dire need of a complete overhaul as the property had fallen into an extreme state of disrepair. Both the hall and the associated stable block are grade II* listed buildings. 
- GENUKI: Llanstephan
- The National Archives | National Register of Archives | Family and Estate details | Archive Details
- Wales Archives Network Wales - Plas Llanstephan Additional Papers
- "Nesta Donne Philipps and others". Peerage (The). Retrieved 7 December 2009.
- Carmarthenshire and Ceredigion; by Thomas Lloyd, Julian Orbach, Robert Scourfield
- "Listed Buildings in Llansteffan, Carmarthenshire, Wales". British Listed Buildings. Retrieved 22 December 2013.
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