The New York gallery Alfred Stieglitz opened in 1905 and later used to exhibit Georgia O'Keeffe's work was 291.
The gallery was originally called the Little Galleries of the Photo-Secession and was located at 291 Fifth Avenue in Manhattan. Because of its address, it became known simply as 291. Stieglitz used the space to promote photography as a fine art and to introduce American audiences to European modernism.
O'Keeffe's drawings were first shown there in 1916 after Anita Pollitzer brought them to Stieglitz's attention. The exhibition helped establish her professional reputation in New York, even though she was then teaching in Texas and had not yet moved into the city's art scene full time.
The gallery closed in 1917. Stieglitz later operated The Intimate Gallery and An American Place, so those names are associated with him too. They are not the answer to this question: O'Keeffe's first Stieglitz exhibition took place at 291.