Denbigh Infirmary was given a rating of one star out of five by inspectors, meaning "major improvement" is necessary.
Betsi Cadwaladr University Health Board said it was "very disappointed" to receive the low rating.
Welsh Conservative Shadow Minister for Health, Darren Millar AM, said the low score "beggars belief".
"Under no circumstances should a hospital be scoring this poorly," he said.
"It is an abysmal result and patients and staff will be rightly alarmed.
"For the hospital to score this lowly, there are serious concerns over food safety. That's not acceptable anywhere - but for our hospitals - it beggars belief."
In a statement Betsi Cadwaladr University Health Board said it was "very disappointed" with the rating and it took rapid action to address the concerns raised.
"An environmental health officer visited the hospital kitchen on the 29 April 2015 and confirmed that we had complied with all the required actions within the notice," the health board said.
"We are now waiting for a re-inspection of the catering facility," it added.
All food outlets must display their food hygiene rating by law.
Earlier this week the catering unit at Glan Clwyd Hospital in Denbighshire received a food hygiene rating of two stars out of five, which means "improvement is necessary".
Health officials said they have since addressed issues at the Bodelwyddan general hospital.
A Denbighshire community hospital has been criticised following an "abysmal" food hygiene rating.