New York's Metropolitan Museum of Art (Met) said it had fulfilled its "dream" of acquiring an important photographic collection.
The collection was a partial gift to the museum
The 8,500-strong archive collected by industrialist Howard Gilman "helped define" the history of photography.
It includes some of the most iconic and classic works from the 19th century and early 20th century.
Photography curator Malcolm Daniel, said: "It's the pre-eminent private collection in this area."
"It's been the dream of the Met to acquire it," he added.
"It has things that were already the textbook examples from the history of photography but at the same time it helped define that history,"
The archive includes photographs from the 1860s British portraitist Julia Margaret Cameron and Roger Fenton, best known for his Crimean War pictures.
It also features American Civil War pictures by Mathew Brady, Alexander Gardner and Timothy O'Sullivan, as well as work by Henri Cartier-Bresson and Man Ray.
The collection was put together between 1977 and 1997 by Gilman, the chairman of the Gilman Paper Company, who together with his curator Pierre Apraxine sought out iconic and classic works.
Daniel said the foundation that owned the collection had made a partial gift of it to the museum, but did not reveal how much the museum had paid for the rest.
Around 20 pictures from the collection would go on display on 17 April, said Daniel.
"Every exhibition we do from now on will always have the Gilman collection because it becomes the essential core of the Met's collection," he added.