Sir William James Thomas, 1st Baronet (10 March 1867 – 3 January 1945) was a Welsh industrialist and philanthropist. He was knighted in 1914 and created a baronet on 10 May 1919, under the title Baronet Thomas of Ynyshir.
The grandson of a coalowner in the Rhondda Fach, James Thomas (died 1901), he lost both parents (Thomas and Jane Thomas) as a child, and was brought up by his grandparents. He inherited his grandfather's fortune, and became a director of the Great Western Railway, Barry Docks, and numerous other industrial concerns. He married Maud Mary Cooper (died 1952) in 1917, and they had three children:
In 1914, Sir William James Thomas sold his coal interests to United National Collieries Ltd, and thereafter made substantial donations to charitable causes, particularly medical causes. Among other things, his donations were instrumental in the establishment of the Welsh National School of Medicine, the first medical school in Wales.
He commissioned the large painting The Care of Wounded Soldiers by Margaret Lindsay Williams for Cardiff Royal Infirmary. He appears in the painting.