291 Gallery exhibited Georgia O'Keeffe's work early in her career.
The gallery was founded and run by Alfred Stieglitz at 291 Fifth Avenue in New York City. Its formal name changed over time, but it became widely known simply as 291, after its street address. The space showed avant-garde European and American art as well as photography.
In 1916, Stieglitz displayed a group of O'Keeffe's charcoal drawings there after seeing them through Anita Pollitzer. This exhibition was a crucial public introduction for O'Keeffe, who was then teaching art in Texas and had not yet established herself in New York.
The gallery closed in 1917, but Stieglitz continued promoting O'Keeffe at later venues, including the Intimate Gallery and An American Place. The answer is sometimes confused with An American Place, the gallery he opened later in 1929. Those were separate Stieglitz galleries.