A two-hour time limit will be imposed on patients using the free car park at Wrexham Maelor Hospital, with those staying longer facing a £70 fine.
The new rules have been brought in to try to stop people abusing the free parking for shopping and commuting.
Patient groups welcomed the free car parking but warned many hospital appointments can run over two hours.
But the hospital assured patients that if this happened, any fine would be cancelled.
The new regulations came into force on Friday as the hospital removed its car parking charges.
It is part of a Welsh Assembly Government initiative to scrap parking payment at 120 of Wales' 130 NHS hospitals and units.
To deter motorists from misusing the car park at Wrexham Maelor, a new maximum stay of two hours with no return within the following two hours, will apply.
Drivers with a 'blue badge' disabled parking permit will be able to park for three hours.
"But the targets of A&E are four hours (to treat a patient) so two hours isn't going to be enough as far as these people are concerned"
The hospital said it hoped this would make the car parks less convenient for rail commuters and people working at the adjacent technology park or visiting the town centre.
People parking at the hospital will still need to display a valid parking ticket, available from machines on the site, to show what time they arrived, although there will not be any charge for this.
Anyone who parks longer will have to pay a £70 fine, reduced to £50 if paid within 14 days.
But Michael Summers, vice-chairman of the Patients Association, warned that there could be problems if people's appointments run over the two hour time limit.
"First of all it's a good start. Some of the charges (for hospital car parks) have been outrageous and something needed to be done with that," he said.
"But the targets of A&E are four hours (to treat a patient) so two hours isn't going to be enough as far as these people are concerned. People there for treatment don't know how long they will be. Some people have to wait for a consultant to be free."
The hospital acknowledged this could be a problem but said patients or visitors who needed to stay at the hospital longer could contact the car park staff, either directly or through the ward or department where they are being treated.
Any penalty notices that have been issued will be cancelled where the patient has evidence of being on official hospital business, or where this is confirmed by the ward or department, it said.
Miles Timperley, general manager for estates at the hospital, said: "Of course we don't want genuine patients or visitors worrying if their appointment runs over or they are delayed for any reason, and our parking staff will be able to exercise discretion."
Charges were scrapped at 120 hospitals and health units in Wales on 1 April after being called "at best an inconvenience and at worst an unfair expense" by Health Minister Edwina Hart.
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