Grand Duke William IV of Luxembourg, Guillaume Alexandre (April 22, 1852 - February 25, 1912) was the eldest child of Grand Duke Adolf of Luxembourg. He became Grand Duke when his father died on November 17, 1905.

On June 21, 1893, he married Maria Ana of Portugal . The couple had six daughters, the eldest of which, Marie-Adélaïde (1894-1924), was declared Heir Apparent on July 10, 1907, and eventually became the first reigning Grand Duchess of Luxembourg. The second daughter Charlotte (1896-1985), then succeeded her on the throne in 1919.