Sophia Dorothea of Brunswick-Lüneburg-Celle (15 September 1666 – 13 November 1726) was the repudiated wife of future King George I of Great Britain.

Sophia Dorothea is best remembered for her alleged affair with Count Philip Christoph von Königsmarck that led to her being imprisoned in the Castle of Ahlden for the last thirty years of her life.

Born in Celle on 15 September 1666, Sophia Dorothea was the only surviving daughter of George William, Duke of Brunswick-Lüneburg, by his morganatic wife Eléonore Desmier d'Olbreuse (1639–1722), Lady of Harburg, a French Huguenot noblewoman.

By contract signed on 22 August 1675, and in open violation of his previous promise never to marry, George William declared that Éléonore was his lawful wife, and a second wedding ceremony was held at Celle on 2 April 1676.

[2] Twenty-two days later, on 24 April, Eléonore began to be addressed at court as Duchess of Brunswick and Sophia Dorothea became legitimate.

By an agreement signed on 13 July 1680, the Lüneburg ruling family recognised Éléonore as Duchess of Brunswick and Sophia Dorothea as Princess of Brunswick-Lüneburg-Celle, with all appertaining rights of birth.

Explaining her reluctant support for the marriage, Duchess Sophia wrote to her niece Elizabeth Charlotte, Duchess of Orléans: "One hundred thousand thalers a year is a goodly sum to pocket, without speaking of a pretty wife, who will find a match in my son George Louis, the most pigheaded, stubborn boy who ever lived, who has round his brains such a thick crust that I defy any man or woman ever to discover what is in them.

Having done his duty, George Louis acquired a mistress, Melusine von der Schulenburg, and started to neglect his wife.

His parents asked him to be more circumspect with his mistress, fearful that disruption of the marriage would threaten the payment of the 100,000 thalers he was to receive as part of Sophia Dorothea's dowry and inheritance from her father.

Around 1690, Sophia Dorothea was reunited with the Swedish Count Philip Christoph von Königsmarck, whom she had known in her childhood when he was a page at the court of Celle.

Contemporary sources show that Sophia Dorothea and Königsmarck were presumed to have had a sexual relationship since March 1692, though she consistently denied it throughout the rest of her life.

[6] Countess Clara Elisabeth von Platen, a former mistress of Elector Ernest Augustus, had tried in January 1694 to persuade Königsmarck to marry her daughter Sophia Charlotte, but he had refused.

It appears that he was killed on the orders, tacit or direct, of George Louis or his father the Elector, and that his body, weighted with stones, was thrown into the river Leine.

Four of Ernest Augustus's courtiers are said to have committed the murder, and one of them, Don Nicolò Montalbano, received 150,000 thalers, about one hundred times the annual salary of the highest-paid official.

Louis then sent agents to Hanover, but they could shed no more light on the mystery than King Augustus II of Poland and Saxony, who spent weeks searching for his missing general.

Both the Emperor and the Elector Frederick III of Brandenburg exerted pressure on Augustus II, but the Polish envoy continued his investigation and even told Count von Platen that Königsmarck had either been captured or killed on the orders of his wife the Countess, out of jealousy.

When his affair with Sophia Dorothea was about to become public, Königsmarck handed their love letters to his brother-in-law, the Swedish Count Carl Gustav von Löwenhaupt.

On 28 December 1694, the dissolution of the marriage was pronounced, and Sophia Dorothea was named as the guilty party for "maliciously leaving her husband".

She was detained in the north wing of the castle, a two-story half-timbered building, and guarded 24 hours a day by 40 men at arms, five to ten of whom were on duty at any one time.

When Sophia Dorothea's father was on his deathbed in 1705, he wanted to see his daughter one last time to reconcile with her, but his chief minister, Count Bernstorff, objected, claiming that a meeting could lead to diplomatic problems with Hanover.

Sophia Dorothea is remembered for a significant act of charity during her imprisonment: after a devastating fire of Ahlden in 1715, she contributed considerable sums of money towards the rebuilding of the town.