Indoor meetings of more than six people from an extended household will be illegal in Wales from Monday.
The rule will not apply to children aged 11 and under, and up to 30 people from different households will still be able to meet outside.
Ministers said the new restrictions followed an increase in cases linked to people meeting and socialising indoors.
Up to four households have been able to form an extended household and meet up since late August.
Until now there has been no legal limit on how many people could gather.
The new restriction will apply in pubs and restaurants, as well as private homes.
It comes after people in Merthyr Tydfil and Rhondda Cynon Taff were asked to wear masks in indoor public spaces and in work, and only use public transport if it was essential.
The new rules will not apply in Caerphilly county borough, which was put under local lockdown on Tuesday, where all extended household meetings are currently banned indoors.
First Minister Mark Drakeford will confirm the changes as part of his review into Wales’ coronavirus restrictions at a press conference on Friday.
The move comes after other parts of the UK also restricted meetings to six – but did so both indoors and outdoors.
The UK government is restricting gatherings in England to just six people from Monday, with some exceptions.
The number of people that can meet in Scotland has also been cut to six.
Wales’ coronavirus rules mean people can only be part of a single extended household, which cannot be changed once arranged.
Despite the gradual easing of restrictions over the summer, meetings indoors with anyone outside your extended household have remained illegal.
The recent rise in cases seen in Caerphilly county borough was blamed on people socialising in other people’s homes in large numbers.
The area has seen one of the highest rates of cases in the whole of the UK, with 91.1 cases per 100,000 people over seven days.
On Thursday, residents of Rhondda Cynon Taff and Merthyr Tydfil were asked to follow a range of measures to avoid a local lockdown.
People have been asked to work from home if they can, not to visit care homes, and wear face masks in indoor or crowded public spaces.
Merthyr Tydfil has the second highest coronavirus case rate in Wales, after Caerphilly.