The 29 officials have signed an open letter to their members declaring that the party needs a “radical and credible” person in charge.
The surprise move comes just weeks after Unite boss Len McCluskey announced that the union was supporting Jeremy Corbyn.
Significantly, many of the stewards backing Mr Smith represent workers with Babcock and BAE Systems – defence companies heavily dependent on the renewal of Britain’s Trident nuclear submarines.
While Mr Corbyn opposes the UK’s nuclear deterrent, his challenger is a strong supporter.
Other firms represented in the letter include Rolls Royce, Siemens and Vector Aerospace Helicopter Services.
The letter says, “Britain needs a Labour Government to defend jobs, industry and skills and to promote strong trade unions.
As convenors and shop stewards in the manufacturing, defence, aerospace and energy sectors we believe that Owen Smith is the best candidate to lead the Labour Party in opposition and in government.
Owen has made clear his support for the industries we work in. He has spelt out his vision for an industrial strategy which supports great British businesses: investing in infrastructure, research and development, skills and training.
He has set out ways to back British industry with new procurement rules to protect jobs and contracts from being outsourced to the lowest bidder.
But Owen has also talked about the issues which affect our families and our communities.”
The shop stewards add, “The Labour party is at a crossroads. We cannot ignore reality – we need to be radical but we also need to be credible – capable of winning the support of the British people.
We need an effective Opposition and we need a Labour Government to put policies into practice that will defend our members’ and their families’ interests. That’s why we are backing Owen.”
The letter exposes deep splits within Unite, where Mr McCluskey faces a battle to be re-elected as general secretary. He has also been criticised for deciding not to ballot his members before endorsing Mr Corbyn.
The shop stewards’ support is also a timely boost for Mr Smith, who bookies make a distant second favourite to be named leader on 24 September.