Sybil Margaret Thomas, Viscountess Rhondda, DBE (née Haig; 25 February 1857 – 11 March 1941) was a British suffragette, feminist, and philanthropist.

[1] 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.

On 27 June 1882 she married David Alfred Thomas, a wealthy Welsh industrialist who later became Liberal Member of Parliament for Merthyr Boroughs.

In the 1890s Sybil Thomas became president of the Welsh Union of Women's Liberal Associations, which was strongly feminist and pro-female suffrage.

He died shortly afterwards and Lady Rhondda devoted the rest of her life to feminist and philanthropic projects.