West Tyrone MLA Catherine Kelly has become the fourth Sinn Féin official to resign over a failure to return money from a Stormont emergency Covid fund.
It had emerged that three Sinn Féin offices received £10,000 payments from the fund, which have since been repaid.
Ms Kelly was a signatory for an account into which £10,000 was lodged earlier this year.
Sinn Féin President Mary Lou McDonald apologised for the " clear failure to immediately reimburse public money".
Stormont's emergency Small Businesses Grant Support Scheme automatically sent out the payments to bank accounts, but it emerged that some of the accounts which received the money were ineligible.
Political party offices were not eligible to receive the cash, which was aimed at supporting small business at the beginning of the first lockdown in March.
In a statement following her resignation, Ms Kelly said: "I have been a signatory to that account for a number of years and as such had a responsibility to ensure it was operated to the highest standard.
"That did not happen in this case."
She apologised "unreservedly to the party, to those who vote for us and to the wider public".
Her party president said the failure to return the grant immediately was "unacceptable".
"Catherine fully accepts that she did not discharge her duties as a public representative in this regard," she said.
Three other party colleagues, the former Foyle MP Elisha McCallion and two other Sinn Féin officials, resigned earlier this week.
In a statement, Ms McDonald said she had accepted the resignation of Ms Kelly.
"The incorrect lodgement of small business grants into three Sinn Féin accounts came to the attention of the Sinn Féin leadership over the course of Monday and Tuesday last.
"The party's examination of this matter is now complete. As leader I once again acknowledge and apologise for the clear failure to immediately reimburse public money."
The party said the funds received by three of its offices had been returned to Land and Property Services, which administered the grant.
But Ms McDonald said the money was only paid back on Monday and Tuesday of this week, months after they were received.
Earlier this week the Department for the Economy revealed that 52 owners of wind turbines and one owner of an anaerobic digester had also received the payment.
Figures released by the Department for the Economy (DfE) show that 24,700 payments were processed under the Small Businesses Grant Support Scheme.
There were 452 payments, totalling more than £4.5m, which were made to those who may not have been eligible.
The DUP, SDLP, Ulster Unionist Party and Alliance have all said that none of their MPs or MLAs received money from the scheme.
On Thursday it emerged that the landlord of a DUP office received a payment from the fund.
DUP leader, and first minister, Arlene Foster said anyone who received money that should not have, must return it.
"It [the scheme] was put in place to help businesses that were struggling at that time, so there is no way politicians or landlords should have been in receipt of that money.
"They have a moral and legal duty to pay that money back," she said.