Sybil Margaret Thomas, Viscountess Rhondda, DBE (née Haig; 25 February 1857 – 11 March 1941) was a British suffragette, feminist, and philanthropist.
She was born in Brighton, the daughter of George Augustus Haig, a merchant and landowner from Pen Ithon, Radnorshire, Wales, and his wife, Anne Eliza Fell. Her father was of Scottish descent and was a cousin of Douglas Haig.
On 27 June 1882 she married David Alfred Thomas, a wealthy Welsh industrialist who later became Liberal Member of Parliament for Merthyr Boroughs. Their principal residence was Llanwern, Monmouthshire.
In the 1890s Sybil Thomas became president of the Welsh Union of Women's Liberal Associations, which was strongly feminist and pro-female suffrage. She was also a prominent moderate in the National Union of Women's Suffrage Societies. Her sisters Janetta and Lotty were also prominent suffragettes and both went to prison for acts of violence in the name of the cause. Her daughter, Margaret Haig Thomas, became one of the most prominent British feminists of the inter-war years. Under their influence, Sybil joined the more militant Women's Social and Political Union. In 1914 she was sentenced to one day's imprisonment after holding a public meeting outside the Houses of Parliament.