​Timothy O'Sullivan (1840-1882) was an American photographer who remains best known for his powerful images of the American Civil War and landscapes of the American West in a body of work that captures "the spirit of nineteenth-century America's national tragedies and aspirations." Although relatively unknown during his lifetime, today he is regarded for his early contributions to American documentary and landscape photography.

While O'Sullivan's early life remains mostly unknown, by 1860, he worked in the renowned studio of Mathew Brady (1823-1896) under the photographer Alexander Gardner (1821-1882). When Gardner left Brady's gallery in 1862, O'Sullivan soon joined him. Both Brady and Gardner employed O'Sullivan to photograph the American Civil War (1861-65). His moving depictions of the grim aftermath of Bull Run, Gettysburg, and Appomattox remain some of the best-known images of this conflict. In addition to his potent battlefield scenes, he also made striking portraits of Union troops and photographed camp life, military fortifications, and the fields and towns that the soldiers passed through. Gardner published many of O'Sullivan's images, which included 44 of the 100 photos chosen for Gardner's seminal album Photographic Sketch Book of the War (1866).

After the war, O'Sullivan and Gardner parted ways. His experience using bulky photographic equipment and a makeshift darkroom in the field earned O'Sullivan a position as expeditionary photographer for Clarence King's Geological Survey of the Fortieth Parallel (1867-69; 1872), an early governmental survey of the American West. During the 1870s, O'Sullivan also briefly documented a Navy Department survey of present-day Panama in search of a canal route (1870) before joining the Wheeler Surveys of Nevada, Utah, and California (1871; 1873-74). The carefully-composed photographs he made on these expeditions convey the majesty of the American West's extraordinary geologic sites, including the Grand Canyon and Shoshone Falls (represented in TMA's collection with Canyon de Chelly, 1873), and Indigenous people and historic sites. He returned to Washington DC in late 1874, where he continued to print his expedition negatives and pursue his own commercial business. After several years of failing to find consistent work, he was appointed the chief photographer of the US Treasury Department in 1880. Unfortunately, soon after accepting the post, he discovered that he had tuberculosis and died in 1882.

After his death, O'Sullivan was virtually forgotten for nearly 60 years until famed landscape photographer Ansel Adams (1902-1984) rediscovered his images of the West in an album acquired from a friend at the Sierra Club. Considering them "extraordinary," he offered to loan the album to Beaumont Newhall (1908-1993) for his groundbreaking Photography: 1839-1937 (1937) exhibition at the Museum of Modern Art. Its inclusion in this and subsequent shows helped revive interest in O'Sullivan's work and elevate his status from merely one of the many photographers employed by Brady and Gardner during the Civil War to a pioneering figure in early American documentary and landscape photography. As such, many survey exhibitions include his work, and in 2010 the Smithsonian American Art Museum mounted a major retrospective of his work that focused upon his Western expedition photographs.

Today, O'Sullivan's work is represented in many museum collections, including the Metropolitan Museum of Art, New York; Museum of Modern Art, New York; Philadelphia Museum of Art; J. Paul Getty Museum, Los Angeles; San Francisco Museum of Modern Art; Nelson Atkins Museum of Art, Kansas City; Amon Carter Museum of American Art, Ft. Worth; National Gallery of Art, Washington, DC; and the Library of Congress, Washington, DC.