MQL (Manchester Quays Ltd) has signed the deal conditional on planning consent and plans to redevelop the 13.5 acre site in a mixed-use desintation.
Planning permission is to be applied for once the remaining ITV and Coronation Street staff relocate to ITV's new base at MediaCity.
Michael Ingall, chief executive at Allied London, said: "We have considered the future of the ITV estate on several occasions and our long standing relationship with ITV has enabled us to acquire what we believe to be an exceptional real estate opportunity.
"We have taken control of over 800,000 sq ft of existing mixed use real estate with various options for future occupation. The estate extends to some 13.5 acres and has development potential of in excess of 3 million sq ft, which we intend to bring forward in partnership with Manchester City Council.
"Our vision is to promote the creation of a completely new mixed-use quarter for Manchester City Centre, at the River Irwell, bounded by Quay Street and Liverpool Road, and produce a masterplan that integrates the established areas within the site with proposed new development.
"This area of the city is rich in heritage, history and character, and we intend to make sure this makes a valuable contribution to our thinking on design and development. We will seek to protect the heritage assets and ensure this whole area is developed in a holistic and sensitive way.
"Manchester has seen a number of high quality developments in the last cycle, which we will seek to match and exceed. At the heart of our plans is the intention to create a new sense of place and community."
Sir Richard Leese, leader of Manchester City Council, added: "We welcome this announcement about a key strategic site in the city and are pleased to be part of this partnership. We look forward to working with Allied and ITV to deliver comprehensive development which will contribute to our growth objectives for the city while respecting the important heritage aspects of the area."
Coronation Street is still filmed at the site with construction on the new set at MediaCity set to continue until the end of 2013.
Jones Lang LaSalle worked on the deal.
Bob Dyson, chairman North West at Jones Lang LaSalle, said: "This represents a good deal for Manchester and signifies a real step forward in the city centre's on-going development. Allied London's pedigree in the city, the site's location close to Spinngfields, and the council's role as both an enabler and advocate for regeneration, means their partnership is ideally placed to open up the next chapter in the area's history."