It said hundreds of jobs in construction will be supported with the redevelopment of Clarendon House in Adelaide Street in the centre of Belfast.
The move will be funded "at no cost to the ratepayer", as the council intends selling off a number of surplus buildings.
Planning approval is already in place for 100,000sq ft of office space.
Clarendon House was originally built in the 1970s and was bought by Cookstown-based McAleer and Rushe last year.
It will be the most significant office development in the city for 10 years, according to the council.
The site is close to city hall and its other offices, the Cecil Ward building on Linenhall Street.
Stephen Surphlis of McAleer and Rushe said: "This deal comes at time of rising demand for Grade A office stock within Belfast and hopefully will act as a catalyst for other office projects in the city."
Recently, the Hastings Group bought the Windsor House office for £6.5m and plans a hotel development with offices on other floors.
Belfast City Council is buying new offices in a deal worth £24.7m.