Éléonore Desmier d'Olbreuse (3,[1] 7[2] or 9 January 1639[3] – 5 February 1722), was a French noblewoman, who became firstly the mistress and later wife of George William of Brunswick, Duke of Lauenburg and Prince of Celle.

She also managed to arrange good marriages to her sisters: the older, Angélique (died 5 October 1688) married Count Henry V Reuss of Untergreiz (1645-1698) in 1678, while the younger, Marie, became the wife of Olivier de Beaulieu-Marconnay (1660-1751), also from a Huguenot noble family, who held high-ranking court office in Hanover.

Despite the fact that George William not only secured a dower for Éléonore but also bequeathed all of his private fortune to her and undertook to take care of her impoverished relatives,[8] she wanted to be recognized as a Duchess of Brunswick with full rights.

[10] Twenty-two days later, on 24 April, the second marriage was made public and Éléonore officially addressed as Duchess of Brunswick and their daughter declared legitimate.

The wedding took place on 21 November 1682 but since the beginning the union was a complete failure: the feelings of hatred and contempt that Sophia of the Palatinate had over her daughter-in-law were soon shared by her son George Louis, who was oddly formal to his wife.

When Sophia Dorothea began a relationship with Count Philip Christoph von Königsmarck and threatened with the scandal of an elopement, the Hanoverian court, including not only George Louis's brothers and mother but also Éléonore, urged the lovers to desist, but to no avail.

On the morning of 2 July 1694, after a meeting with Sophia Dorothea at the Leineschloss castle, von Königsmark was seized and disappeared, being presumed murdered at the instigation of George Louis, and his body thrown into the Leine river.

When George William was on his deathbed in 1705, he wanted to see his daughter one last time to reconcile with her, but his Prime Minister, Baron Andreas Gottlieb von Bernstorff raised objections and claimed that a meeting would lead to diplomatic complications with Hanover; the ailing duke no longer had the strength to prevail against him.

Sophia Dorothea unsuccessfully asked her former husband one last time that he should let her leave Ahlden to live with her mother in complete seclusion, but her request was denied.