More than 1,300 homes will be built on the garden site
Work to restore the formal gardens at the International Garden Festival site is expected to start in November, Liverpool City Council said.
The site, on the banks of the River Mersey, has been derelict for 25 years.
The council's executive board gave the go-ahead to developers Langtree to carry out the work, which should see the gardens reopened in a year.
The Northwest Regional Development Agency has offered a £2.1m grant to Langtree for restoration work.
However, in order to accept the grant changes in a development agreement between the city council and the developers had to be agreed. This included deferring a dowry, to cover maintenance costs, from Langtree for five years.
In addition to the development agency cash, the firm is also under consideration for a further £1.6m from the Northwest European Regional Development Fund (ERDF).
Council leader Warren Bradley said the gardens "should be something the city is proud of and a real asset to the community".
He added: "We want to make sure that, having secured public funding for the site, that no obstacles are put in the way of the restoration of the green spaces so we were happy to make the necessary amendments to our development agreement."
Fell into disrepair
The areas which will be restored include the Chinese and Japanese gardens, lakes and associated watercourses and the woodland sculpture trails.
The Land Restoration Trust will be responsible for the management and maintenance of the park once the works are completed.
Plans for the £250m redevelopment, which also includes more than 1,300 homes, were approved after a public inquiry last year.
The 1984 International Garden Festival was billed as a "five-month pageant of horticultural excellence and spectacular entertainment".
But after the festival was over, the site changed hands a number of times and fell into disrepair.