A further 200-odd spaces will be available on the site by the summer, in addition to around 320 already promised by the Southern Health and Social Care Trust.
At a hospital board meeting on Monday, chief executive Colm Donaghy pledged that a total of 500 extra spaces will be available by the summer.
Parking at the hospital has been an issue of contention in recent months, with patients and visitors becoming increasingly frustrated by the lack of spaces.
Just last week, the Portadown Times featured the story of a man who was blocked in by another driver, while trying to take his young daughter home after an anaesthetic.
And weeks before that, a cancer patient told how she was unable to find a space in order to visit her elderly mother.
Health watchdog the Southern Health and Social Care Trust has also criticised the shortage of parking at the hospital, branding it "inconvenient and dangerous".
Speaking at Monday's meeting, Mr Donaghy said the new psychiatric unit is due to open in early May, creating 150 new spaces.
Work is also being finalised on a further 167 spaces at the back of the hospital, with new signage currently being prepared and roads and footpaths upgraded in preparation for the extra traffic.
OFFICIAL
An official announcement on the siting of the remaining 200 spaces is expected to be released shortly.
Mr Donaghy added that once the new car parks were up and running, the main road up to the hospital from the roundabout would become a non parking zone.
At the moment, motorists are forced to park along both sides of the approach road, due to the lack of parking spaces.
He said he was very aware of safety issues surrounding parking along the road, not only for pedestrians but for ambulances.
ambulance
Said Mr Donaghy, "I know there was an incident in which one of our ambulances was impeded coming onto the site because two buses were trying to pass each other."
The issue of safety was raised by several board members with non-executive director Alistair Joynes saying there were "significant " issues to do with safety, and the risk from crossing roads, that needed to be addressed urgently.
Chairman Anne Balmer pointed out that the Southern Health and Social Care Trust was just one-year-old and had many resources to allocate in this period of time. "This time last year there were no extra parking spaces available, This year we will have 500 extra," she said.
The trust is also looking at the possibility of introducing pay car parks, and is currently awaiting guidance from the Health Minister on this.