Thomson, who is currently managing director of New London Architecture, will take up the position in July.
"Huge privilege to join RIAS"
She will become chief executive of the Royal Incorporation of Architects in Scotland (RIAS), which is the professional body representing chartered architects in Scotland.
"It is a huge privilege to join the RIAS as chief executive, and I am looking forward to moving back to Scotland to take up this exciting role," said Thomson.
"I am relishing the opportunity to work with my new RIAS colleagues to support and celebrate the work of RIAS members in Scotland and around the world."
The chief executive role was recently created to be the most senior non-elected position at the organisation. Previously RIAS' most senior non-elected employee was Neil Baxter, who held the title of secretary and treasurer.
Baxter resigned from his position at RIAS in 2017 after more than 150 architects accused the organisation of financial mismanagement. Karen Stevenson has been acting secretary since Baxter's departure.
Appointment "part of a programme of change"
According to RIAS, Thomson's appointment comes at a time when the organisation is refocusing.
"The appointment of Tamsie Thomson as chief executive is part of a programme of change at the RIAS that will see the organisation expanding its outreach and influence, and demonstrating the importance of architecture to Scotland's economy, society and culture," said the organisation.
"I am a passionate advocate of architecture's capacity to improve people's lives, and believe that the RIAS has a huge role to play through its work with members, clients, decision-makers and the public," added Thomson.
"We have a shared purpose to inspire and influence, and to demonstrate the critical role of architects and architecture to Scotland's cultural, social and economic life."
Thomson grew up in Edinburgh, where RIAS is based, and will return to the city for the role.
She led the London Festival of Architecture as director from 2016 to 2019 and became managing director of New London Architecture in 2019.
Previously she worked for seven years at the Royal Institute of British Architects where she was regional director of RIBA London.