The office's Dutch embassy building in Berlin pipped entries from British finalists Norman Foster and Future Systems to win the 2005 European Union Prize for Contemporary Architecture.
The embassy was singled out for its 'extraordinary relationship' with its surroundings. It was also commended for the 'quality of the urban reflection and intelligence of the concept implemented, especially as regards the unprecedented concept of trajectory and the new potential it brings to this project of great complexity'.
The jury, which included Aaron Betsky, Mohsen Mostafavi, Eduard Bru and Francis Rambert, was chaired by Zaha Hadid, who won the competition two years ago with her Terminus Hoenheim-Nord in Strasbourg, France.
Koolhaas will be presented with the cheque at the Mies van der Rohe Pavilion in Barcelona tonight (Monday 11 April).
Meanwhile, the Emerging Architect Special Mention award went to fellow Dutch designer NL Architects for its BasketBar at the University of Utrecht.