Purnell will replace Tessa Jowell in the role, and is one of many new faces in Brown's first act as the new Prime Minister.
RIBA head of public affairs Steven Harding said: 'As a personality he is exciting, bright likeable and experienced.
'He has a genuine interest in architecture, and was essential to persuading then Prime Minister Tony Blair to throw his weight behind the Better Public Buildings programme.'
And in a thinly veiled attack on Jowell, Harding added: 'The RIBA will be looking forward to working with him, and looking forward to seeing a Secretary of State bringing energy and intelligence to the role.
'We have now got someone who will be certainly sympathetic towards architects and architecture.'
Jowell has been named as Minister for the Olympics, London and the South East in the reshuffle.
Other movements in the shake-up were Hazel Blears replacing Ruth Kelly as Communities Secretary - Kelly is now the new Transport Secretary - and the appointment of Yvette Cooper as Housing Secretary.
Cooper will be joined by her husband and Brown darling Ed Balls, who will take over from Alan Johnston as Education Secretary.