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Amelia Osborne, painted by François-Hubert Drouais

Amelia Osborne, Marchioness of Carmarthen and de jure 12th Baroness Darcy de Knayth and 9th Baroness Conyers and 5th Countess of Mértola (12 October 1754 – 27 January 1784) was a British peeress and a Portuguese countess.

Amelia was the only child of the 4th Earl of Holderness and his wife, Mary. On 29 November 1773, she married Francis Osborne, Marquess of Carmarthen and they had three children: Lord George William Frederick, Lord Francis Godolphin (later Baron Godolphin) and Lady Mary Henrietta Juliana (who married the 2nd Earl of Chichester).

Lord and Lady Carmarthen divorced in May 1779 and a month later, Amelia then married John "Mad Jack" Byron (the father of the poet, Lord Byron) and they had one daughter, Augusta. A year earlier, Amelia had inherited the right to her father's baronies of Darcy de Knayth and Conyers (although this wasn't confirmed until 1798, long after her death) and also inherited the Portuguese countship of Mértola from him. On her own death in 1784, they were inherited by her eldest son, George.