Visiting will not be allowed on the affected wards to try to stop the outbreak spreading further.
Six wards are still affected at Wrexham Maelor and one at Mold Hospital in Flintshire.
Last week, the Maelor was closed to all visitors apart from relatives of seriously ill patients.
Admissions to the hospital and all outpatients departments continue to run normally.
Some wards at Chirk Hospital in Wrexham were also closed last week but these have since reopened.
Betsi Cadwaladr University Health Board said a daily review is in place and the public will be informed when the restrictions are lifted.
The main symptoms of the norovirus infection are sudden nausea, stomach ache followed by severe "projectile" vomiting and / or diarrhoea.
Symptoms usually begin between 12 and 48 hours after a person becomes infected.
Hospital visitors are asked to check with the nurse in charge of a ward before visiting.
The wards affected at Wrexham Maelor are Evington, Erddig, Bedwen, ENT, Onnen, and Morris Ward.
The wards affected at Ysbyty Gwynedd are Glyder, Moelwyn, Aran, Glaslyn, Cynan and Aneuryn Ward.