Patients, visitors and staff at NHS sites across north Wales are being "actively encouraged" to wear face masks after a spike in Covid-19 cases.
Betsi Cadwaladr health board said the number of cases across north Wales was low but Wrexham had seen an increase.
It applies to those visiting public spaces for appointments, surgery or emergency treatment at Glan Clwyd, Ysbyty Gwynedd and Wrexham Maelor.
It urged people not to visit Wrexham Maelor Hospital A&E unless "necessary".
The face mask advice also applies to other premises managed by the health board, including community hospitals and GP surgeries.
From Monday, three-layer face coverings will be mandatory on all public transport in Wales - buses, trains, and taxis.
Welsh Government guidance currently says that while doctors, nurses and care providers should wear medical masks while treating patients, patients attending outpatients appointments and visitors do not need to wear face coverings.
But in England hospital visitors and outpatients have been required to wear face coverings since rules were changed on 15 June.
To date, 375 people in the Betsi Cadwaladr health board area, which covers the whole of north Wales, have died with coronavirus - the highest number of Wales' seven health boards.
Wrexham is the local authority area with the second highest number of cases per 100,000 of the population in Wales, behind Merthyr Tydfil.
In the last seven days, 45 new positive cases of coronavirus have been confirmed in Wrexham, with 18 reported in Flintshire.
But Gill Harris, executive director of nursing and midwifery, said while the number of confirmed cases across north Wales was currently low, Wrexham had seen an increase.
It comes after 237 cases were linked to Rowan Foods factory in the town."Covid-19 is highly infectious and it can be challenging to prevent the spread of the virus and although we have taken a range of actions, we appeal to the public for their support as we all have a role to play in preventing the spread of infection," she said.
"We ask that our communities continue to maintain good hand hygiene and observe social distancing guidance."
Ms Harris said there would be more details on face mask use within north Wales NHS sites in the coming days.
Responding to the new advice, the Welsh Conservatives said visitors and outpatients to hospitals in all other parts of the UK were required to wear face masks and Wales was "lagging behind"."Time and time again, we are seeing Wales being held back by the constant dithering and delay from this government," member of the Senedd (MS) for South Wales Central Andrew RT Davies said."The Welsh Conservatives call on the Welsh Labour-led Government to finally put Wales first, and make face coverings mandatory for all visitors and outpatients immediately."
But the Welsh Government said the most effective way for people to protect themselves from coronavirus was to follow social distancing rules and to wash hands regularly.
"Face coverings are not a substitute for these measures," a spokesman said.
Public Health Wales, which is managing the response to the outbreak, said while wearing face coverings was "not mandatory" it encouraged people to wear them "where possible".
"Public Health Wales supports the statement from Betsi Cadwaladr University Health Board earlier today, actively encouraging patients, the public and staff to wear face coverings or masks in public areas of all Health Board sites," a spokesman said.
What is the Welsh Government's advice on wearing face masks?
But in Wales, this is not the case, and face masks are only "advised" in crowded places where social distancing is not possible.
At the outset of the pandemic, Wales' chief medical officer Dr Frank Atherton said there was only a "marginal public health case" for wearing face masks.
But both he and the Welsh Government always said they would keep the measure under review - and if the evidence changed, so would change the advice.
From Monday 27 July three-layer face coverings will be mandatory on all public transport in Wales - buses, trains, and taxis.
It brings Wales into line with Scotland, England and Northern Ireland, who all demand passengers wear face masks.
But the first minister has remained firm on face coverings for shopping or other activities - it is not happening in Wales.
"The advice of the Welsh Government is that if places are crowded then face coverings are advisory. Where places are not crowded, it is a matter for the individual citizen to make that decision," he said.