Theresa Yelverton (née Maria Theresa Longworth; c. 1827–33 – 13 September 1881) was an English writer who became notorious because of her involvement in the Yelverton case, a 19th-century Irish law case, which eventually resulted in a change to the law on mixed religion marriages in Ireland.

A Roman Catholic convert, she subsequently returned to her father's house in Smedley, where they fell out over religion.

[2] She first met Major William Charles Yelverton, who from 1870 was 4th Viscount Avonmore, while aboard a steamer on the English Channel in August 1852.

Theresa was staying there, with a friend who was another Catholic convert and a Sister of Charity, Arabella, daughter of Charles Macfarlane.

Theresa heard of this marriage three days later, and on 30 June a Catholic cleric showed Ramsay a copy of the certificate of the August 1857 Irish ceremony.

[10] The immediate consequence was that Theresa applied to the Edinburgh procurator fiscal, and Yelverton was put in Calton Jail on a bigamy charge.

"Theresa was alternately vilified and celebrated, portrayed as a victim who had been 'mercilessly abandoned' and accused of being a lascivious seducer.

Francis Farquhar wrote that she "spent the summer of 1870 in Yosemite, where she attached herself to the Hutchings family and made eyes at John Muir.