Duchess Therese Petrovna of Oldenburg (30 March 1852 – 19 April 1883) was the youngest daughter of Duke Peter Georgievich of Oldenburg and his wife Princess Therese of Nassau-Weilburg.

On 12 May 1879, Therese married Prince George Maximlianovich Romanovsky (1852–1912), youngest son of Maximilian de Beauharnais, 3rd Duke of Leuchtenberg and his wife Grand Duchess Maria Nikolaevna of Russia. Therese's elder brother Duke Alexander Petrovich had been married to George's sister Princess Eugenia Maximilianovna since 1868. Through either the deaths or morganatic marriages of his elder brothers, George was the head of the Russian branch of the House of Beauharnais.

Therese's grandfather had married Grand Duchess Catherine Pavlovna, daughter of Paul I of Russia, and their descendants had been raised in Russia ever since and become completely "Russianized", much like George's own family. Thus despite her German title, Duchess Therese, like her brother Duke Alexander and their father before them, had grown up entirely in Russia. Therese and her siblings were always considered a part of the Russian imperial family.

Therese and George had one son, HIH Alexander Georgievich, 7th Duke of Leuchtenberg (13 November 1881 - 26 September 1942), who would later feature in many 1909 newspapers after rumors spread that he entered into a morganatic marriage with American Marjorie Gould. In 1912, Alexander was reported to have gained the reluctant consent of Emperor Nicholas to marry the wealthy commoner, Marriane Friedlander Fuld, but only on the condition that the union would be considered unequal, with none of his titles being passed onto his wife or possible children. Despite being the senior descendant of Eugène de Beauharnais (son of Empress Joséphine), Alexander was far from rich, and served as a captain of the Russian Hussars of the guard and as an aide-de-camp to the Emperor. He was, however, the principal heir to his grandfather.

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