Work on the Regatta Quay development started in 2007 but its developer went into administration.
The 20-storey building had been called an eyesore by local residents.
John Howard, from new owners Marina Developments Limited, said: "The completion of this project will breathe new life into the area."
Builders should be on site within the end of year, but the project will take "two or three years" to complete, the company said.
Work on the building will be completed "largely in accordance with the original planning consent", administrators Baker Tilly said.
It will include about 150 flats and will have commercial units on the ground floor.
Nigel Miller, from Baker Tilly, said: "There are still a few loose ends to tie up before the sale is able to complete."
David Ellesmere, leader of the Labour-run Ipswich Borough Council, said: "This is fantastic news for the town and hopefully will kick start the regeneration of the rest of the waterfront.
"It will also be good news for creating and securing local construction jobs."
Ipswich MP Ben Gummer, a Conservative, said completion of the building was "vital" for the town's regeneration.
"The building at present is not only an eyesore but is a waste of valuable space," he said.
An unfinished Ipswich building dubbed "the wine rack" since work ceased in 2009 could be completed, as a sale has been agreed.