Marie-Adélaïde, Grand Duchess of Luxembourg (Marie Adélaïde Theresia Hilda Antonia Wilhelmina vu Lëtzebuerg; June 14, 1894 – January 24, 1924) was a daughter of Grand Duke Guillaume IV of Luxembourg and Marie Anne of Portugal. Her maternal grandparents were Miguel of Portugal and Adelaide of Löwenstein-Wertheim-Rosenberg.
Eldest of six sisters, she was proclaimed Heir Apparent on July 10, 1907, to solve the succession crisis. Thus, when her father died on February 25, 1912, she became the first reigning Grand Duchess of Luxembourg. She was also the first sovereign of Luxembourg since 1296 to have actually been born within the country.
She was highly interested in politics and took an active part in the government and the political life of the Grand Duchy. During World War I, she enjoyed a rather cordial relationship with the German occupiers, for which she was harshly criticized after the end of the war. Although not having done anything unconstitutional, voices in Parliament began to demand her abdication in January 1919. At the same time, prominent political figures in both neighbouring France and Belgium espoused annexationist plans towards the Grand Duchy and thus had a vested interest in discrediting Marie-Adélaïde. After consulting with the Prime Minister, she abdicated on January 14,1919, and was succeeded by her younger sister Charlotte.
Marie-Adélaïde entered a convent in Italy, taking the name "Sister Marie of the Poor", and died at Hohenburg Castle of influenza. She is interred in the Ducal Crypt of the Notre-Dame Cathedral in the city of Luxembourg.