The building, designed by Lifschutz, Davidson, Sandilands and opened in the autumn of 2013, has also been awarded a RIBA Regional Award 2015.
This adds to Bonhams' success in winning the BCIA (British Construction Industry Awards) 2014 Building Project of the Year (£10-50m) and the American Institute of Architects (UK) Design Excellence Award 2014.
The Bonhams building stands on the site at 101 New Bond Street occupied by the company's previous HQ and main London saleroom.
Working within tight planning constraints, the architects created a building with three large flexible auction rooms (two of which are double height), a temperature controlled environment, and lighting that can be changed at the touch of a button.
Bonhams chairman Robert Brooks said: "I am immensely proud that the building has won such a high profile award. Our ambition was to create a space that was a pleasure to be in and that would set the scene for the objects we sell. It is very gratifying to have recognition from such a prestigious institute that we have succeeded in this aim."