Sir Rhys Rhys-Williams, 1st Baronet, DSO, QC, DL (20 October 1865 – 29 January 1955), born Rhys Williams, was a British Liberal Party politician from Wales. He later left the Liberal Party for the Conservatives.

Rhys-Williams was the son of Judge Gwilym Williams and Emma Eleanor Williams JP. His wife Juliet Rhys-Williams (née Glyn) was a writer and prominent Liberal politician who, like her husband, later joined the Conservative Party via the Liberal Nationals. They met in 1919 when Juliet Glyn began working for Rhys-Williams as his private secretary during his period of office as parliamentary secretary at the Ministry of Transport. They married on 24 February 1921. They had two sons and two daughters. Their son, Sir Brandon Rhys-Williams became a Conservative MP and Member of the European Parliament.

Rhys-Williams was educated at Eton College, which he entered in 1880, and Oriel College, Oxford.

In the First World War he served in the Welsh Guards, reaching the rank of lieutenant-colonel. He saw active service, was wounded, was twice mentioned in dispatches and won the DSO. In 1915 he served for a year as Acting Military Attaché at the British Legation in Tehran. He ran an intelligence service for the Russians in their campaigns against the Turks and was awarded the Order of St. Vladimir with the Swords by the Czar in 1916. In 1917 Rhys-Williams was attached the War Office to act as Assistant Director-General Movements and Railways.

Williams was called to the Bar at the Inner Temple in 1890 and joined the South Wales Circuit. He also practised in the Parliamentary Committee Rooms. He took silk in 1913. In 1906 Rhys-Williams became Chairman of Quarter Sessions in Glamorgan in succession to his father. From 1922, Rhys-Williams served as Recorder of Cardiff.

Williams was first returned to Parliament as Member of Parliament (MP) for Banbury at a by-election in September 1918 and was returned unopposed at the 1918 general election as Coalition Liberal. On his appointment as Recorder of Cardiff he was obliged to resign from Parliament and fight a by-election on 22 June but he was again returned unopposed. He served on until the 1922 general election when he stood in a different seat and was defeated.

In 1919 Rhys-Williams was appointed parliamentary secretary to the Ministry of Transport under the direction of the Minister of Transport Sir Eric Geddes. However he soon fell out with Geddes and resigned just two months after being appointed.

In 1923, Rhys-Williams was approached by the Aberavon Liberal Association to stand as their candidate in the forthcoming general election in opposition to Labour leader Ramsay MacDonald but he declined. By the 1930s, Rhys-Williams had become more anti-socialist in stance. Never a great party man, in March 1931 he tried to get the Liberal candidate at the by-election at Pontypridd, Captain G Crawshay, to make a public statement that as soon as the Labour government introduced what he described as any openly socialist measure he would vote to try to turn out the government. Rhys-Williams promised Crawshay that if he would make this pledge he would do his best to get the Conservatives to stand aside and support him in a straight fight against Labour. When Crawshay refused to agree Rhys-Williams came out in support of David Evans, the Conservative candidate in the by-election. In the event the seat was easily retained by Labour, with Crawshay in second place and Evans third.