The neglected landmark will be reborn as a cultural venue in June when it is taken over for 10 days in June.
Spaces in and around the A-listed building will become home to music, theatre, dance, spoken word and visual art after the city council agreed to allow it to be used for the award-winning event.
An outdoor music and a pop-up open-air bar overlooking the Old Town will also be created under plans for the event, which will kick-start a summer which will see Edinburgh’s festivals celebrate their 75th anniversary.
Specially-commissioned Hidden Door shows will be performed “in-the-round” in the debating chamber originally created in the Regent Road building in the 1970s when it was proposed as a home for a future Scottish Parliament.
A temporary stage will be built in the heart of the main auditorium, which will have a capacity of around 250, while the entire site will be able to accommodate up to 1000 festivalgoers. More than 200 volunteers are expected to help clean-up the site and create the festival infrastructure.
Hidden Door - which reopened Leith Theatre in 2017 for the first time in nearly 30 years and took over part of the former Granton Gasworks last year - will be staged at the old Royal High School before work gets underway on a long-awaited refurbishment of the building.
Councillors approved plans to create a national music centre, as well as home for St Mary’s Music School in Edinburgh, in October following a protracted battle with developers of a luxury hotel project over the future of the site.
While Hidden Door’s line-up will be kept under wraps until next month, the programme is expected to include groups who will be using the new national music centre, including St Mary’s Music School.
The school was built on a site overlooking the Old Town on Regent Road between 1826 and 1829, to a design by architect Thomas Hamilton.
The festival, which will be staged from 9-18 June, will be the most significant use of the building since the school relocated from the site in 1968.
The main debating chamber was used for an opera in 1998, by the Edinburgh Art Festival in 2014 and for a theatre production in 2016.
However Hidden Door has pledged to create work for four main spaces inside the building, as well as encourage audiences to explore its “myriad of passageways, corridors, nooks and crannies.”
Admission will be free to the festival site during the day with the all-ticket evening events expected to run until 11pm or midnight.
David Martin, Hidden Door’s creative director, said: “Our very first event, at the Market Street vaults, looked onto this building.
“Once we had built up a good relationship with the council they actually suggested we have a look at it.
“We were thinking of using it when we went for Leith Theatre but have had it in mind since then and we were actually on the verge of announcing plans to use it in 2020 when lockdown happened.
"The biggest difficulty we had with the Granton Gasworks was not quite knowing what the restrictions were going to be at the time.
“We think the Covid landscape will be different this summer, but we still have to plan with it in mind.
"The old Royal High will be much more about the inside spaces, but the outside spaces will be important too, including the terraces looking out across the city.
“There will be multiple things happening at multiple times across the entire site – it will be a bit like a micro-Fringe.
“The debating chamber is the most charismatic space in the building. We love the in-the-round nature of it, but the space in the middle is a bit too small, so we’re going to build a stage on top of it, almost like a boxing ring."
Mr Martin said the festival was inviting “Scotland’s newest, most daring artists, dancers, theatre-makers, poets and musicians to take over such a prestigious building that has been dark for so long and flood it with creative energy.”
He added: “The reason we decided to do Hidden Door at the start of the summer is because the festival is about new stuff and up-and-coming artists.
“We are working with bands who will be collaborating with artists, theatremakers and dancers to create spectacular over-top-performances in the debating chamber, which will feel quite different to the music performances on the main stage.
"There will always be things happening in our various performance spaces, which very much fits with how people experience Hidden Door. They like to come along and do their own thing.
“We're really excited about creating what we believe will be the best bar in Edinburgh. We’ll bring people through the old school gates onto a terrace overlooking the city, but we’ll also have bars throughout the building.”
Kenneth Taylor, headteacher at St Mary’s, said: “It’s great to be involved in this celebration of the breadth and diversity of artistic and creative practice alive in Scotland, and a real opportunity for our young musicians to share their contribution to this vibrant and engaging scene.
“The setting of the old Royal High School is a perfect canvas for the imagination - opening doors to memories of its past and excitement about its future – especially as it will become the home for our pupils."
Donald Wilson, culture convener at the city council, said “Following the success of Hidden Door festival at Granton Gasworks last year, we’re delighted to support the festival once again and that the site of the old Royal High School can be brought into use for this year’s festival.
“It will be a fascinating insight into this iconic building and its historic interiors. I look forward to the full programme being announced.”