The Theodore Roosevelt Cyclopedia is a comprehensive project to publish, in one collection, the significant sayings, important conversations and writings (less his letters) of the 26th President of the United States, Theodore Roosevelt. Originally conceived by Dr. Albert Bushnell Hart, a history professor at Harvard University, a personal friend of Roosevelt and member of the Roosevelt Memorial Association, now known as the Theodore Roosevelt Association, Hart's goal was, in his words, to "present in alphabetical arrangement, extracts sufficiently numerous and comprehensive to display all the phases of (Theodore) Roosevelt's activities and opinions as expressed by him." An A-Z online index of the original work is also maintained by the TRA. The 1941 Cyclopedia is out of print, but was made available in a CD-ROM version in 1989, the 1989 version can be found online.

Theodore Roosevelt, the 26th U.S. president, popularly known as "TR" and "Teddy" (although Roosevelt despised that name), died on January 6, 1919. Within a few days, the Roosevelt Memorial Association was founded by Roosevelt's friends and associates. The association was formally chartered by special act of Congress, May 31, 1920,

to perpetuate the memory of Theodore Roosevelt for the benefit of the people of the United States of America and the world....

Led in the years 1919–1957 by secretary and director Hermann Hagedorn (1882–1964), poet, author, historian, friend and biographer of Roosevelt, the association engaged in a wide spectrum of programs and activities to preserve his memory.

In the association's Annual Report 1924, Hagedorn announced that "a Roosevelt Cyclopedia or Roosevelt Thesaurus" was being edited by Professor Albert Bushnell Hart of Harvard University. Albert Bushnell Hart (1854–1943) was a classmate of Roosevelt's at Harvard, Class of 1880, and like Roosevelt, a Phi Beta Kappa. Hart received a Ph.D. degree at the University of Freiburg in Germany in 1883, and that same year joined the faculty of Harvard. He taught at Harvard 1883–1926. One of the first generation of professionally trained historians in the United States, a prolific author and editor of historical works, Albert Bushnell Hart became, as Samuel Eliot Morison says, "The Grand Old Man" of American history, looking the part with his "patriarchal full beard and flowing moustaches." Hart was a devoted friend and follower of Theodore Roosevelt, and was elected as a Roosevelt delegate to the Republican convention of 1912.

He became an enthusiastic trustee and supporter of the Roosevelt Memorial Association, and said that from the time of Roosevelt's death he had the idea of editing a Roosevelt-centered cyclopedia. The projected reference work would, Hart explained, "present in alphabetical arrangement extracts sufficiently numerous and comprehensive to display all the phases of Roosevelt's activities and opinions as expressed by him." He wrote Hagedorn: "What we are after is the crisp, sharp, biting sparks that flew from the Roosevelt brain." Hart told the survivors of the Harvard Class of 1880 that editing the cyclopedia "will be a very interesting and agreeable service to the memory of our great classmate."

But from the beginning the project was plagued with problems. Hart's time was taken up with other commitments. He was editor of the American Year Book, 1926–1932, edited a five-volume history of Massachusetts in 1927–1930, and worked as the official historian of the George Washington bicentennial commission in the 1920s and 1930s. Hart had to postpone the cyclopedia, and asked the association for research and clerical staff.

But the executive committee of the Roosevelt Memorial Association delayed appropriations for the cyclopedia, because the expense was "so great," and it was not until May 1928 that a budget was approved for the cyclopedia, although the project had been publicly announced years before. Finally, in 1931 Hart presented a rough draft of the cyclopedia to Hagedorn. But the book needed much more work. By now the elderly Hart "began to decline," wrote Samuel Eliot Morison; and Hagedorn reported to the RMA Executive Committee that Hart could not finish the project "because of his advanced years."