Sebastian Tombs will chair Architecture and Design Scotland from the beginning of July and will split his time between his current and new job from next week.
Tombs said: 'Leaving the RIAS after 19 years will be a wrench - and I will miss my many friends and colleagues there.
'But I am looking forward to taking Architecture and Design Scotland into a brighter future for the country,' he added.
Tombs is an architect who worked in Philadelphia, US, before settling in Scotland in 1975. He has worked for the RIAS since 1986.
Architecture and Design Scotland is a public body set up to inspire better quality in design and architecture.
It will take on the design review role of the Royal Fine Art Commission for Scotland (RFACS) when it comes into effect on 1 April.