The archive consists of around 150,000 photographs and includes unique coverage of the work of Denys Lasdun, Norman Foster and James Stirling, as well as special projects such as Donat's depiction of the old buildings of Holloway Prison prior to their demolition in the 1970s.
The RIBA's photographs curator Robert Elwall said he was delighted that the institute had obtained the collection.
'This is a tremendous coup for the British Architectural Library and the RIBA Trust, we are immensely grateful to the Donat family for making such an important accession possible,' he said.
Born in 1933, Donat studied at the Architectural Association before working for a short period in the Schools Division of the London County Council.
He was increasingly drawn to photography, principally as a result of trips to Turkey, Iran (1956), and Crete (1960 and 1961) with fellow students Peter Ahrends, Richard Burton and Paul Koralek.