Michael Fallon said the date for cutting the first steel would help secure new investment and safeguard hundreds of skilled jobs until 2035.
He also announced that a contract for two new offshore patrol vessels would be signed shortly.
This will secure jobs before the Type 26 frigate work is fully under way, he said.
An £859m deal to build the ships on the Clyde was signed in February 2015.
But the project has been scaled back and hit by repeated delays, with concerns that jobs could be lost as a result.
'Value for money'During a visit to the Govan area of Glasgow, Mr Fallon said: "Backed by Britain's rising defence budget, the Type 26 Programme will deliver a new generation of cutting-edge warships for our Royal Navy at best value for taxpayers.
"The UK government's commitment today will secure hundreds of high-skilled shipbuilding jobs on the Clyde for at least two decades and hundreds more in the supply chain across Britain."
The defence secretary also announced a £100m contract with the consortium MBDA to deliver the Sea Ceptor missile defence system for the ships.
Gary Smith of the GMB union said it was "fantastic news" for the upper Clyde shipyards - though he said the UK government had not delivered on all its promises.
He told the BBC's Good Morning Scotland programme: "We were told there was to going to be a 'frigate factory' built on the Clyde that would allow us to deliver ships more efficiently.
"It would perhaps have given us the facility to build ships that could be exported.
"And Michael Fallon in truth was dragged kicking and screaming to this announcement today after we exposed the fact that his government did plan to move some of the work originally planned for the Clyde, down south.
"But cutting through it all great, news for the workforce and great news for the economy in greater Glasgow as well."
The promise of new Royal Navy orders to secure the Clyde shipbuilding industry was made before the Scottish independence referendum in 2014.
The number of planned new frigates was later scaled back from 13 to eight, although the MoD retained the option to build five smaller and cheaper general-purpose vessels.
The Type 26 Global Combat Ships will be built by BAE Systems at the company's Glasgow yards in Govan and Scotstoun.
For more see the BBC website at http://www.bbc.co.uk/news/uk-scotland-37861162
Of course that is from the official press release which will not mention all the Government interference and dodging that has gone on over the past 18 months or so and the word on the street is that they even intended to shift some of the work if not all of it down South, but nothing new in that, it would appear that for now at least the shipbuilders (the few that remain) will have work for the foreseeable future which has to be regarded as a good thing, although the number of ships has been cut from lucky "13" down to eight this is still a very large project which now has the few remaining yards around Britain all crying out for their bit of the pie.
This may or not come about with a release of news as it gets closer to election time that the 5 remaining smaller ships will indeed be built elsewhere, for now it is looking good for shipbuilding in the two remaining main shipyards on a river that could at one time boast of close to Fifty (50) shipyards on this relitivly small stretch of water.