US President Franklin D. Roosevelt is believed to have had a romantic connection with which European Royal?

Märtha Sofia Lovisa Dagmar Thyra, known as Princess Märtha of Sweden (28 March 1901 – 5 April 1954) was Crown Princess of Norway as the spouse of the future King Olav V from 1929 until her death. Olav became king in 1957, so that Märtha never became Queen of Norway. Princess Märtha had on elder sister, Princess Margaretha of Denmark, a younger sister, Queen Astrid of Belgium, and a niece Grand Duchess Joséphine-Charlotte of Luxembourg. Her son, Harald V, is the current (2025) king of Norway.

In 1940, Märtha escaped from occupied Norway to her home country, Sweden, and was then evacuated to America by President Franklin D. Roosevelt. She effectively advocated for Norway and did fundraising until the end of the war. Crown Prince Olav, however, accompanied his father to the United Kingdom with the Norwegian government-in-exile. The Norwegian royal couple, like many other couples during that time, were separated for much of the war.

In August 1941 Märtha travelled with Roosevelt aboard the presidential yacht and sailed to Newfoundland and the Atlantic Charter meetings with Winston Churchill. Roosevelt's son James stated "There is a real possibility that a true romantic relationship developed between the president and the princess." Novelist Gore Vidal wrote that Märtha was Roosevelt's "last love". Roald Dahl, then a fighter pilot assigned to Washington, agreed that all the smoke indicated a real fire: 'The President has it in his mind that he would like to sleep with her."