James Wycliffe Headlam (1863-1929) was a British academic historian and classicist, who became a civil servant and government advisor. He changed his surname to Headlam-Morley, in 1918. He was knighted in 1929 for his public service.
An influential figure, he worked on propaganda in World War I, and after it was over was involved in the drafting of the Versailles Treaty. He effectively sponsored Arnold J. Toynbee for appointment in 1924 to Chatham House. He also gathered materials on the diplomatic history of the origins of WWI, as an official production of the British government, and contributed to it, though the main editor was Harold Temperley. Historian Anna Cienciala attributes to Headlam and Sidney Edward Mezes, an academic and advisor to Woodrow Wilson and Executive Director of the Inquiry group, the 1919 proposal to make Danzig a free city.
He was the younger brother of Arthur Cayley Headlam (1862-1947), the bishop and author.
He married in 1893 Elisabeth Charlotta Henrietta Ernestina Sonntag (1866-1950), a German musician and composer, usually known as Else Headlam-Morley. The historian Agnes Headlam-Morley (1902-1986) was their daughter.