Sir Bertram Clough Williams-Ellis, CBE, MC (28 May 1883 – 9 April 1978) was a Welsh architect known chiefly as the creator of the Italianate village of Portmeirion in North Wales. He became a major figure in the development of Welsh architecture in the first half of the 20th century, as well as working on commissions across the UK and Ireland, in a variety of styles and building types. He also campaigned widely for the preservation of rural England and Wales, for which he was knighted.

Clough Williams-Ellis was born in Gayton, Northamptonshire, England, but his family moved back to his father's native North Wales when he was four. The family have strong Welsh roots and Clough Williams-Ellis claimed direct descent from Owain Gwynedd, Prince of North Wales. His father John Clough Williams Ellis (1833–1913) was a clergyman and noted mountaineer while his mother Ellen Mabel Greaves (1851–1941) was the daughter of the slate mine proprietor John Whitehead Greaves and sister of John Ernest Greaves.

He was educated at Oundle School in Northamptonshire. Though he read for the natural sciences tripos at Trinity College, Cambridge, he never graduated. After a few months at the Architectural Association School of Architecture in London in 1903–04 (which he located by looking up "Architecture" in the London telephone directory), he worked for an architect for a few months before setting up his own practice in London. His first commission was Larkbeare, a summer house for Anne Wynne Thackeray in Cumnor, Oxfordshire, in 1903-04 (finished 1907) which he designed while still a student.

In 1908 he inherited a small country house, Plas Brondanw, from his father, which he would restore and embellish through the rest of his life, as well as rebuilding it after a fire in 1951.

Williams-Ellis served with distinction in the First World War, first with the Royal Fusiliers and then with the Welsh Guards as an intelligence officer attached to the Tank Corps. He was described as lieutenant on the day of his wedding.

After the war, Williams-Ellis helped John St Loe Strachey (later his father-in-law) revive pisé construction in Britain, building an apple storehouse followed by Harrowhill Copse bungalow at Newlands Corner using shuttering and rammed earth. The building was featured in a book he wrote on the subject of earth construction.

One of his earliest designs of 1905 was for a pair of Welsh labourers' cottages in a vernacular style with end gable chimneys which imitate the 16th-century Snowdonia Houses In 1909 he designed a house in an advanced Arts and Crafts style for Cyril Joynson at Brecfa in Breconshire In 1913–1914 he was responsible for the rebuilding of Llangoed Hall in Breconshire, one of the last country houses to be built before the First World War. While it is a mixture of a number of historic styles, it also has modern features with elements such as the chimneys derived from the work of Lutyens. Other work in Wales by Clough Williams-Ellis includes the Festiniog Memorial Hospital of 1922, Pentrefelin Village Hall, and the Conway Fall Cafe.

In 1925, Williams-Ellis acquired the land in North Wales that would become the Italianate village of Portmeirion (made famous in the 1960s as the location of the cult TV series The Prisoner, and the 1976 Doctor Who story The Masque of Mandragora). Portmeirion is notable not only as an architectural composition, but also because Clough Williams-Ellis was able to preserve fragments from other now demolished buildings from Wales and Cheshire. These include the plaster ceiling from Emral Hall