About 100 construction jobs will be created when work starts and it will employ 60 staff once open.
Developer Signature Living said the building “once again will become the jewel in Cardiff’s crown”.
The redevelopment should take 18 months and the first phase is expected to open later this year.
The project will see all its original features restored and a heritage area dedicated to its history created.
The deal ends years of uncertainty surrounding the future of site and the firm said it hoped its investment would further boost the regeneration of Cardiff Bay.
Signature Living co-founder, Lawrence Kenwright, said the firm’s plan was a “credible and serious” one which was “hugely exciting for Cardiff”.
“The Coal Exchange is a magnificent landmark building. Our plan to regenerate it will make it something that Cardiff can be proud of once again,” he said.
Ian Hill, of campaign group Save the Coal Exchange, said members were excited about the plans but were keen for work to “start immediately, to stop further degradation”.
He added: “The Cardiff Coal Exchange building is vast, which will take a considerable sum to develop in its entirety.
“The worry is that work will begin but to the parts in the best condition first, as they will bring the fastest return on investment, risking the future of the building once again.”
Cardiff council said the works were subject to relevant planning and listed building consents.
The Coal Exchange – a short history
- Dates back to 1883 when Cardiff was at the centre of the coal trade
- Coal merchants, ship owners and their agents met daily on the trading floor
- At one time the price of the world’s coal was determined there and in 1904 the world’s first recorded £1m deal was struck there
- Closed in 1958 after the decline of the coal industry in Cardiff
- earmarked as a future home for the Welsh Assembly but that plan fell through after devolution was rejected in a 1979 referendum
- In more recent years it has been used for concerts, dinners and other events
- There were once plans for the exchange hall to be restored and a 1,300-seat banqueting hall, public square, office space and innovation centre developed