Historic England have awarded a £108,400 grant to Wakefield Council to enable the port, which was a controlled entry point to a fort or prison, to be opened up to visitors.
It is part of the £3.5m Key to the North Project to restore and conserve the heritage site.
Neil Redfern, principal inspector of ancient monuments for Historic England said: “Pontefract Castle has played a pivotal role in the history of England.
"From struggles to claim and keep the English crown to suffering the longest siege of any place in the 17th century Civil Wars, Pontefract Castle is very much the key to the North.
“Over the past two years it has also been one of the country’s largest conservation projects, which Historic England has been very pleased to support.
"The current work to stabilise the Sally Port will ensure this fascinating part of the castle can be seen and enjoyed by the public for the first time in hundreds of years.”
Work on the Sally Port was scheduled as part of the Key to the North Project and the area has been excavated.
But during excavations, it was discovered that a retaining wall, thought to be there, did not exist and the work ground to a halt.
Historic England stepped in with the grant to enable the structure to be made stable and accessed for the first time since 1649.
Coun Jacquie Speight, Wakefield Council's cabinet member for culture, leisure and sport, said: “We are very grateful to Historic England for the additional grant, which will cover 80 per cent of the cost of building a retaining wall.
"Work had to stop on the Sally Port once we realised it was unstable and before we were awarded this funding it looked as though the only option would be to fill the area back in.
“Thanks to Historic England visitors will still be able to access the Sally Port once work to the monument is completed at the end of this year.”
The remaining 20 per cent of the cost is coming from the £3.5m project fund. But in order to balance the books, the council said a planned staircase into the Sally Port will no longer be built.
The latest repair grant brings the Historic England contribution to the project to more than £600k.
The Heritage Lottery Fund (HLF) has contributed £3.04 million with additional funding from Wakefield Council, the Wolfson Foundation and landfill charity EpaC.