Llantarnam Abbey is a Grade II*-listed house of the Sisters of St Joseph of Annecy and the site of a former 12th-century Cistercian abbey located in Llantarnam, Cwmbran, in the county borough of Torfaen in southeast Wales.

It was founded as a daughter house of Strata Florida Abbey, Ceredigion. Llantarnam Abbey took its place among the Welsh Cistercian abbeys as the revival of political power supported the growth of the Order during the twelfth century. Although various dates are suggested for the year of its foundation, it can be safely attributed to have been founded in the last quarter of the twelfth century. It was to remain active for over three hundred and fifty years before the suppression of 1536 finally closed its doors on 27 August 1536. Later that century, the abbey's vast tracts of lands, including the immediate abbey environs, were sold into the hands of the Morgan family. The landscape associated with the abbey stayed largely intact until the development of Cwmbran new town in 1949.

After its dissolution in 1536, William Morgan of Pentrebach bought the property in 1554 and he later became a Member of Parliament (MP) and High Sheriff. The abbey may have been built by his son Edward Morgan, also an MP and High Sheriff, who was repeatedly fined for his recusancy. His son, Sir Edward Morgan, 1st Baronet was a noted Royalist during the Civil War. The second baronet, also Edward, sheltered the Jesuit priest who was executed at Usk in 1679, Saint David Lewis. The house was only intermittently occupied from then until it came into the hands of Reginald Blewitt. He rebuilt it in Tudor Revival style to the designs of Thomas Henry Wyatt in 1834–36. The renovation supposedly cost £60,000 and that expense, coupled with the collapse of his finances, forced Blewitt into exile from 1851 to 1868. After his death a decade later, the nephew who inherited the property sold the abbey in 1895 to Sir Clifford Cory, colliery owner, shipping magnate and Liberal politician, who lived there until his death in 1941. After his death, it became a depot for the American Army during the Second World War. In 1946 it became again a monastic institution, in the hands of the Sisters of St Joseph of Annecy.

The main abbey building was Grade II* listed on 6 June 1962 "as an early and very elaborate Tudor revival country house."

The first notable historical study of Llantarnam Abbey was researched by Joseph Bradney in his multi-volume A History of Monmouthshire from the Coming of the Normans into Wales down to the Present Time. Bradney's work gives a detailed account of its setting within the landscape while giving as complete a historical account as possible through his available sources at the time. A lot of his work concentrated on the post-medieval period including the pedigrees and heraldic coats of arms concerning the major families connected to the abbey. Also covered by Bradney are histories and observations on the granges and associated lands. These include Cwmbran, Pentre-bach, Ty Coch, St Dials and Llandderfel, with detailed notes on St Michael's parish church. The work is still used as a primary source by the Gwent Glamorgan Archaeological Trust when researching planning decisions on land associated with the abbey and its granges.

In 1979 Sr Teresa Mahoney privately published Llantarnam Abbey: 800 Years of History. Historical records from Llantarnam Abbey and its granges are embellished with historical records from many other Cistercian sites around Britain and Europe. Compiled into a single volume, this culminates in an account which is probably the most complete history of the abbey that embraces an overall European Cistercian perspective. Although Mahoney's study takes into account the Welsh political problems that were associated with Llantarnam through its initial patronage, it also addresses the overall British situation that arose through the dissolution of the monasteries.

The most prolific historian of the Llantarnam Abbey site itself is David H. Williams, who read as a historical geographer at Cambridge. After his initial 1964 article, 'Cistercian Abbots in Medieval Gwent', further publications included 'Llantarnam Abbey' (1967); White Monks in Gwent and the Borders; The Welsh Cistercians: Vols I & II; 'The Cistercians in the Middle Ages and Catalogue of Welsh Ecclesiastical Seals as known down to AD 1600'; Atlas of Cistercian Lands in Wales; Catalogue of Welsh Ecclesiastical Seals in the National Museum of Wales I; and 'Gwent Seals: VI'. In 2001, both of Williams’s 1984 volumes were published as one book.

In contrast, research by Madeleine Gray has used a multi-disciplinary approach predominantly concentrating on the outlying granges on Mynydd Maen, Mynyddislwyn and in the Rhondda valley. This included experimental archaeology, which retraced a long and arduous pilgrimage route from Llantarnam Abbey, through the granges of St Dials and Llandderfel, before arriving at Penrhys in the Rhondda; A post-medieval survey of Henry, Earl of Pembroke's manor of Mynyddislwyn includes many aspects of medieval land use deducted from the survey. Gray also examines the political situation in south Wales regarding the establishment of land endowment and the foundation of granges in south-east Wales.