Flood defences costing almost £70m will protect 1,600 homes and 400 businesses over the next century, the government has said.
A £67m barrier is at the heart of the scheme, unveiled in Ipswich by Floods Minister Therese Coffey.
The Environment Agency paid for 80% of the project, which will defend the town from tidal surges up the River Orwell.
Dr Coffey said she was "delighted" to see the government investing in the "future prosperity" of Suffolk.
The 200-tonne rotating steel flood barrier, at the southern end of the New Cut, can be raised in minutes and will be supported by almost a mile of new and refurbished flood walls as well as flood gates downstream on the banks of the River Orwell.
Dr Coffey, who is the Conservative MP for Suffolk Coastal, cut the ribbon to officially open the defences.
The government's most recent climate change projections predict sea levels could rise by up to 1.15m (3.7ft) by the end of the century.
The new defences will give Ipswich a much higher level of protection from the type of North Sea tidal surge which threatened the town in 2007 and 2013.
Emma Howard Boyd, chairwoman of the Environment Agency, said the scheme will "help people and businesses prosper in a more resilient Ipswich".
The scheme has been funded by the Environment Agency (£54.6m), Ipswich Borough Council through the Haven Gateway Partnership (£3.4m), the New Anglia Local Enterprise Partnership (£6.6m), the Regional Flood and Coastal Committee (£1.7m) and UK Power Networks (£1.1m).
It has also freed up hectares of land for regeneration.