Sophia Sidney, Baroness De L'Isle and Dudley (née FitzClarence; 25 August 1796 – 10 April 1837), was the eldest illegitimate daughter of William IV of the United Kingdom and his longtime mistress Dorothea Jordan.

She was married to Philip Sidney, 1st Baron De L'Isle and Dudley, and had four surviving children.

Sophia FitzClarence was born on 25 August 1796 on Somerset Street in London, the eldest daughter of Prince William, Duke of Clarence and St Andrews, by his longtime mistress, the comic actress Dorothea Jordan.

[2][3] While circumstances prevented the couple from ever marrying, for twenty years William and Dorothea enjoyed domestic stability and were devoted to their children.

She was his favourite child and her death caused him intense grief, exacerbated by the fact that he was completely estranged from her brother, the Earl of Munster.