The Welsh Government will put new guidance that states only two households can form a "Christmas bubble" during a five-day period into law ahead of the festive period.
It comes after First Minister Mark Drakeford said that the current picture of coronavirus in the country was "extremely serious" and stated that people should not be using their full allowance of three households over the five-day period.
Speaking at a Welsh Government press conference on Wednesday, Mr Drakeford also said that the country would go in to another lockdown from December 28 because of a "sustained rise in coronavirus cases".
In the press conference, the First Minister said: "Here in Wales, the position is that only two households should come together to form an exclusive Christmas bubble during the five-day period.
"The fewer people we mix with in our homes, the less chance we have of catching or spreading the virus."
Now, after a meeting between government ministers, the new guidance will become law for the five-day period.
Analysis from our Political Editor Adrian Masters
The Welsh Government later moved to harden its guidance into regulations. Welsh ministers decided this afternoon that the two household limit should be enshrined in law.
They were said to be concerned that the clarity of the message would be lost, particularly with much media coverage focussing on the Prime Minister's announcement for England. So they will change the regulations.
It will mean that here in Wales, it will be the law that during the five days of easing, a Christmas bubble can be formed from a maximum of two households and one single person support household.
A Welsh Government spokesman said the decision to make its guidance on limiting Christmas bubbles to two households a legal requirement was to make it "easier" for the public to understand.
"Ministers decided this afternoon that we will be amending the regulations in Wales, so for December 23 to 27 the restriction on two households meeting will be part of Welsh law," the spokesman said.
"It makes it easier, so we don't have the position where the law says one thing and the guidance says something else."
Plaid Cymru Leader Adam Price accused the Welsh Government of "making things up as they go along."
He said: “Public trust is essential in a public health crisis to ensure people stick to the rules and make the health policy work.
"Whilst enshrining the guidance on household mixing at Christmas into law was the right decision they’ve failed to get the public messaging right and shifting back on forth on such a crucial policy within the space of a few hours has done nothing to inspire public confidence. “The sad reality at the moment is that we are lurching from one crisis to another with new lockdowns becoming unavoidable emergency measures due to the failure of Welsh Government policy.
"We will now need additional support for those people and businesses that will be hit hardest by these new restrictions.
"But above all we need a clear and consistent strategy and messaging which regains control of the virus and sets out a roadmap to reopening and recovery, giving people confidence."
The First Minister also announced that Wales would move into stricter level 4 restrictions - the equivalent of a lockdown - from 6pm on Christmas Day.
He announced that non-essential retail and gyms would close at the end of trading on Christmas Eve with hospitality having to close from 6pm on the 25.
These restrictions will then be followed by much stricter rules on December 28 that includes travel bans.
These restrictions will apply to the whole of Wales and will be reviewed after three weeks.