Adam Badeau (December 29, 1831 – March 19, 1895) was an American author, Union Army officer, and diplomat. He is most famous for his service on the staff of Ulysses S. Grant during the American Civil War and his subsequent three-volume biography of Grant. Badeau enjoyed a successful career as a writer, and assisted Grant with the research, fact checking, and editing when Grant authored Personal Memoirs of Ulysses S. Grant. Badeau died in Ridgewood, New Jersey, and was buried at Sleepy Hollow Cemetery.