The Brexit Party has been officially recognised as a group in the Senedd by the presiding officer.
In a letter to the new party's leader in the assembly, Elin Jones said the requirements of the assembly's rules had been met.
Mark Reckless said he was pleased the decision had been made but added he was "surprised" it took five days.
Brexit Party leader Nigel Farage unveiled the new group made up of four former UKIP AMs last week.
The move means Mark Reckless, Mandy Jones, Caroline Jones and David Rowlands will have access to more staff and will be given a greater status in the assembly.
It comes as a YouGov/ITV poll on the European Parliamentary election in Wales suggested the Brexit Party was in the lead with 36% of the vote, putting Labour in third behind Plaid Cymru.
'Fair and impartial'
Group leader Mr Reckless said on Monday: "I would have preferred a decision earlier than five and a bit days after our application."
But he said he was grateful that Ms Jones had applied the rules of the assembly "in a fair and impartial way".
He told a press conference he did not know if the presiding officer had been politicised.
"I'll give her the benefit of the doubt," he said.
Plaid Cymru's group of AMs, and some from Labour, had wanted the assembly's rules altered to stop the Brexit Party group from being formed.
A group of six members have written to the assembly's business committee, which meets Tuesday, asking it to consider changing standing orders so regional AMs can only be members of the political groups they were elected to.
Critics argue the group has no political mandate, with all its AMs having entered the assembly through the UKIP party list.
Mr Reckless said the Brexit Party would "reap the electoral reward" if Plaid and Labour AMs still tried to stop the new group by changing the rules.
"It would be a preposterous course of action," he said.
But he said it was now a "higher hurdle" to change standing orders retrospectively and try to "unmake a group which has already been properly recognised".
'A massive grudge'
The Labour group is also expected to discuss the matter, which would need to be voted on in the Senedd if it went any further, on Tuesday.
Not all of the 29 Labour AMs are thought to support the proposals. One Labour source said changing the rules would give the Brexit Party a "massive grudge that they will dine out on for a long time".
A spokeswoman for the assembly said: "We can confirm that the Llywydd has notified the four assembly members that standing orders have been satisfied in relation to their application to form a Brexit Party group."
'Careful not to rock the boat'
By Aled ap Dafydd, BBC Wales political correspondent
In his press conference on Monday Mr Reckless, having got the OK from the presiding officer, was careful not to rock the boat with her.
But Mr Reckless was vague on when he actually became a supporter of the Brexit Party. I'm left unsure from his answers whether he actually was a paid supporter when he appeared on the steps of the Senedd with Nigel Farage last Wednesday.
Asked repeatedly about his colleague David Rowlands calling far right activist Tommy Robinson "courageous", we had a series of "no comment" answers.
Would Mr Rowlands be disciplined? "No comment".
And could Nigel Farage depend on Mark Reckless - the politician having jumped ship from parties in both Westminster and Cardiff Bay?
He replied: "We both backed Brexit, we have that in common".
An ITV Cymru Wales/YouGov/Cardiff University poll on Thursday's EU election in Wales has suggested the Brexit Party is in the lead - with Labour ranked third and the Tories in sixth place.
The single poll of 1,009 adults put the Brexit Party on 36% of the vote. Plaid Cymru were on 19%, Labour on 15%, and the Liberal Democrats on 10%.
It suggested the Greens would get 8% of the vote, the Tories 7%, UKIP 2%, and Change UK at 2%.
Roger Scully of Cardiff University projected the Brexit Party would win two seats, Labour one and Plaid Cymru one, if the results were replicated at the election.
Meanwhile, on a visit to Cardiff Market on Monday, the shadow Brexit secretary Sir Keir Starmer said he would campaign for remain if a new referendum was held.
Labour has been embroiled in an internal row about whether to support a new poll outright - currently it will only support one if certain conditions are met.
It did not change the stance for the election, despite claims from pro-EU figures in the party that to not do so would mean it would struggle to take on the Brexit Party.
Asked if he would campaign to stay in the EU in a second referendum Sir Keir said: "I would campaign for remain."
When asked what Labour would do, he added: "The Labour party will decide when we see the question".
The European Parliament elections in Wales
There are eight parties fighting for four Welsh seats in the European elections on 23 May.
Welsh Labour, the Welsh Conservatives, Plaid Cymru, the Welsh Liberal Democrats, UKIP and the Green Party are joined by Change UK and the Brexit Party.