The Rent Smart scheme will require landlords to be trained in their responsibilities and obligations.
Latest Welsh Government figures show fewer than 25% had been granted licences by last week.
Only 25,353 out of a possible 100,000 Welsh landlords are registered.
The Welsh Government wants Rent Smart to help tackle bad landlords who give the private rented sector a bad name.
Housing charity Shelter Cymru said nearly a third of its workload came from private tenancies.
Although it can take only a few minutes to register, the process of completing the licensing takes about eight weeks and many have not even started it.
Back in July, 13,000 landlords had registered with Rent Smart but progress has been slow.
The Rental Landlords Association (RLA) fears fewer people will let out houses, meaning there will be fewer houses to rent and that this will push up the cost.
RENT SMART FACTFILE:
Angela Durrant - who rents out a flat in Cardiff Bay - said she only found out about the Rent Smart changes by chance but is unhappy with the amount of work it involves.
"I've actually chosen to go through a letting agent in order to [register] because I was more concerned I'd miss something out or fall foul of the law," she said.
"I was really annoyed because we've respectfully rented out the property for 10 years and I understand it might be trying to root out rogue landlords but it's not really taking into consideration people who are trying to do a good job with perhaps one or two properties."
The Welsh Government said it would raise overall standards and improve the reputation of private rented housing which had been "damaged by the actions of rogue and even criminal landlords and lettings agents".
A spokesman added: "We hope the scheme will ultimately lead to more investment in Wales by improving its reputation".
Meanwhile, 96% of those who had completed the relevant training said "it will make them a better landlord."
There are fears that thousands of residential landlords in Wales will be breaking the law without realising it, with just six weeks to go until all will have to be registered.