Amalie Sophie Marianne von Wallmoden, 1st Countess of Yarmouth (1 April 1704–19 or 20 October 1765) was the mistress of George II of Great Britain from the mid-1730s until his death in 1760. Born into one prominent family in Hanover and wed into another, she became a naturalized citizen of Britain in 1740 and was granted the life peerage title of "Countess of Yarmouth". She was the last royal mistress to be so honored. She remained in Britain until the 1760 death of George II, who is believed to have fathered her second son, Johann Ludwig, Reichsgraf von Wallmoden-Gimborn. She returned to Hanover to live for the nearly five years she survived the king.
She was born Amalie Sophie Marianne von Wendt on 1 April 1704, the daughter of Hanoverian General Johann Franz Dietrich von Wendt and Friderike Charlotte von Wendt (née Von dem Busche). Her aunt was Melusine von der Schulenburg, Duchess of Kendal. She entered into the House of Wallmoden in 1727 with her marriage to Gottlieb Adam von Wallmoden, with whom she shared a son, Franz Ernst von Wallmoden. She was described in 1738 in a letter to Charles, Viscount Townshend as being a brunette with "fine black eyes", "very well shaped, not tall, nor low; has no fine features, but very agreeable in the main."
George II was first attracted to Von Wallmoden during a 1735 visit to Hanover, where she lived with her husband. In 1736, she bore a son, Johann Ludwig, Reichsgraf von Wallmoden-Gimborn, said to be the unacknowledged illegitimate child of the king. By 1738, George II's visits to Hanover to see his mistress were numerous enough to invite satire by Samuel Johnson in the poem "London". The king ended the necessity of those visits after the death of his wife Caroline of Ansbach in November 1737, sending for Von Wallmoden to join him in England, but it did not put an end to Johnson's disapproval. In 1739, Johnson wrote scathingly of the king's relationship with von Wallmoden, "his tortured sons shall die before his face / While he lies melting in a lewd embrace".
In 1739, von Wallmoden divorced her husband. In 1740, she was naturalized and given the non-inheritable title of Countess of Yarmouth, the last royal mistress to be so honored. She was officially designated Amalie Sophie de Wallmoden to obscure the question of her marital status. Horace Walpole indicated that her primary focus was on pleasing the king, though she was also said to be interested in the bestowing of peerages, reputedly playing a part in the creation of a Barony for Stephen Fox-Strangways, 1st Earl of Ilchester in 1741 and in the newly minted title Viscount Folkestone for Jacob des Bouverie in 1747.
After the death of the king on 25 October 1760, she returned to Hanover. She died on the 19th or 20 October 1765.