The announcement was made at a glittering awards bash at the Royal Museum in Edinburgh on Saturday, which was broadcast live on television, presented by Kevin McCloud.
The judges, who included AJ editor Isabel Allen, said of the parliament: 'The building is a statement of sparkling excellence.
'On the Memory Wall [part of the building's multifaceted facade] one of the statements reads: 'Say little and say it well.' This building is definitely saying a lot rather than little but it definitely says it well.'
RIBA president Jack Pringle presented Benedetta Tagliabue, representing the EMBT design team, with a cheque for £20,000.
At the same ceremony, Amin Taha Architects was presented with the AJ First Building Award and a cheque for £5,000 by architect and contractor Robin Ellis for its Gazzano House.
For more in-depth Stirling Prize coverage, visit www.ajplus.co.uk/riba2005