Liberty House said the site at Sheerness in Kent will restart production of bar and rod products by mid-2017, employing scores of workers.
The company said the move broadens the range of steel products it can supply to the UK steel market, including hot rolled coil from it Newport, South Wales works, heavy plate from Dalzell in Scotland, and re-bar and wire rod from Sheerness.
The Kent plant can produce up to 750,000 tonnes a year, taking Liberty's steel rolling capacity to 2.5 million tonnes a year.
VB Garg, chief executive of Liberty Steel Newport, said: "We are particularly excited about the prospect of restarting the Sheerness rolling mills which were closed by the previous owner four years ago.
"Although we originally thought we would move the mills elsewhere, we now believe it will be better to reopen the existing site as it will save time and cost and allow us to take advantage of emerging opportunities in the market."
The company will recruit an initial workforce of 60 people, rising to 100 as market conditions allow.
"We hope some of the former workers will consider coming back but either way we intend to invest in building skills for the future," added Garg.
Community represented the workforce there before the closure and have continued to campaign with the local community for the return of steel to Sheerness.
Responding to the announcement, Roy Rickhuss, general secretary of Community said: “In the midst of the steel crisis, this decision by Liberty House is encouraging news for our industry.
"This announcement is a credit to the resilience and hard work of our members in Sheerness who have fought for years, often with little prospect of success, to bring the steel industry back to their local community.
“Community has represented steelworkers in Sheerness for decades and it was a sad day when production ended four years ago. We will be working closely with Liberty House to bring back the good quality steel jobs that were lost.
“The UK steel industry still faces significant challenges and it is vital that local and national government provide all necessary support in helping bring this steelworks back to life.”