Restrictions in four local authority areas in North Wales — Denbighshire, Flintshire, Conwy and Wrexham — will come in at 6pm on Thursday.
The announcement on Tuesday evening will affect around 504,000 people and will bring the number of people in Wales under lockdown to more than 2.3 million.
It means 16 areas of the country will face restrictions, with the four local authorities joining Caerphilly county borough, Rhondda Cynon Taf, Bridgend, Merthyr Tydfil, Newport, Blaenau Gwent, Cardiff, Swansea, the town of Llanelli, Neath Port Talbot, the Vale of Glamorgan, and Torfaen, all in South Wales.
Health minister Vaughan Gething said: “Unfortunately, we have seen a rise in coronavirus cases in four north Wales local authority areas – in Denbighshire, Flintshire, Wrexham and Conwy.
“These are largely linked to people socialising indoors and are the pattern of transmission similar to what we have seen in South Wales.
“We have worked closely with local authority leaders and the police in North Wales and we all agree about the need to take swift action to control the spread of the virus.”
The rise in cases in the four areas is said to be linked to people meeting indoors, not following social-distancing guidelines, and returning from summer holidays overseas with the virus.
The new restrictions mean people under lockdown will not be allowed to enter or leave their areas without a reasonable excuse, such as travel for work or education.
People will only be able to meet people they do not live with outdoors and will not be able to form, or be in, extended households.
“It’s always difficult to make the decision to impose restrictions but we hope that these measures will make a positive difference – just as we have seen in Caerphilly and Newport, where local residents have pulled together and followed the rules,” Mr Gething said.
“It is important we all work together and support each other. This isn’t just about protecting ourselves, it’s about protecting each other.”
Reporting by PA Media