Leopold III (3 November 1901 – 25 September 1983) was King of the Belgians from 1934 until 1951. On the outbreak of World War II, Leopold tried to maintain Belgian neutrality, but after the German invasion in May 1940, he surrendered his country, earning him much hostility, both at home and abroad.

His act was declared unconstitutional by Prime Minister Hubert Pierlot and his cabinet, who presently moved to London to form a government-in-exile, while Leopold and his family were placed under house arrest. In 1944, they were moved to Germany and then Austria, before being liberated by the Americans, but banned for some years from returning to Belgium, where his brother Prince Charles had been declared regent. Leopold's eventual return to his homeland in 1950 nearly caused a civil war, and under pressure from the government, he abdicated in favour of his son, Prince Baudouin, in July 1951.

Leopold's first wife, Astrid of Sweden, was killed in a road accident while on a driving holiday in Switzerland in August 1935, being much mourned by the public. His second morganatic marriage, to Lilian Baels in captivity in 1941, was not valid under Belgian law, and she was never permitted the title of queen.