Rheinallt Nantlais Williams MBE (1911 – 1993) was a Welsh professor of the philosophy of religion and principal of the Presbyterian United Theological College Aberystwyth in Wales.
Rheinallt Williams was born in Ammanford in Carmarthenshire, one of seven children (two of whom died in infancy) of Nantlais Williams, a Welsh poet and a Presbyterian Christian leader during the 1904-1905 Welsh Revival and his wife Alice Maud Jones, a relative to J. T. Job. Williams gained a 1st Class honours degree in philosophy at Cardiff University, which was followed by research at the University of Edinburgh and the University of Marburg. He was elected Lewis Gibson Scholar at Westminster College, Cambridge, where he studied theology for three years. On leaving Cambridge he became the minister at the Tabernacle in Whitchurch in Cardiff from 1939 to 1949, excluding a period of war service.
During World War II he was appointed a Chaplain to the Forces (4th Class) in May 1942. In September 1943 Captain Williams was appointed MBE (Military Division) for services during the North African Campaign. He spent four years as Professor of Practical Theology at the Theological College in Bala before being appointed Professor of Philosophy of Religion at the United Theological College Aberystwyth in 1953.