With demolition work of the Café Royal wing of the theatre completed earlier this year, foundations have now been laid for the distinctive oval-shaped new addition to the existing building.
It will begin to emerge from the ground over the next few weeks.
When complete, the work will create a dramatic new entrance on the corner of Cowcaddens Road and Hope Street, providing new foyers and accommodating new bars, box office, cloakrooms, toilet facilities and hospitality suites.
The new building will also allow lifts to be installed for the first time, significantly improving access to all levels of the theatre.
The Scottish Government has committed funding of £4m to the development.
Visiting the construction site, Scotland's Secretary for Culture and External Affairs, Fiona Hyslop, said "The Theatre Royal is a beautiful building and one of our nation's major cultural assets. However, it was built in Victorian times and needs updating to meet modern needs.
"This project – which is due to be completed by the Commonwealth Games in 2014 – will transform the theatre.
"The creation of a new education suite and plans to open up the foyer for use during the day will also turn the theatre into a fantastic cultural resource for Glasgow and Scotland as a whole."