Michelle O'Neill has said Arlene Foster must answer questions over the source of the DUP's £435,000 Brexit 'dark money'.
The Sinn Fein Stormont leader urged the DUP to "restore confidence" after claims that the man whose mystery organisation donated the money had links to Saudi intelligence.
She made the call after the DUP's Sir Jeffrey Donaldson said the money was raised legitimately.
It is illegal for parties to accept donations from foreign governments, and electoral law requires parties to carry out due diligence to establish the source of funds.
Writing in the Belfast Telegraph, Mrs O'Neill said "the obvious question" is whether former Saudi intelligence chief Prince Nawwaf bin Abdulaziz Al Saud was the source of the DUP's cash pile.
Mrs O'Neill said Mrs Foster "needs to answer the questions around the Brexit dark money and all the other scandals, which have blighted her party".
But she added that "it seems that Arlene Foster would rather talk about anything but".
The sum of £435,000 was given to the DUP by the Constitutional Research Council (CRC), a little-known Great Britain-based group of pro-Union business figures, ahead of the EU referendum last year.
Part of it was used to buy a four-page advertising supplement in the Metro freesheet in London and other British cities urging readers to vote leave.
But the DUP's political opponents have questions over the original source of the CRC's money.
Sir Jeffrey said: "I believe that they have raised their money legitimately and we were delighted to receive the donation from them for the Brexit campaign."
He told the Open Democracy website: "We have published the details of that, we have complied not only with the law but we have gone much further than we were required."
Former Scottish Conservative Richard Cook chairs the CRC.
Mr Cook founded a company in 2013 with Prince Nawwaf, according to Open Democracy.
Prince Nawwaf is a former director general of the Saudi Arabian intelligence agency and father of the Saudi ambassador to the UK.
Mr Cook has rejected as laughable the suggestion that the group's funding is shadowy and denied that some of it is foreign.
Mr Donaldson said that identifying the sources of the CRC's money was a matter for the CRC, not the DUP. He also told Open Democracy he was not aware of Mr Cook's link to Prince Nawwaf.
However, the Electoral Commission states on its website that "regulated donees can only accept a donation of more than £500 made to them in connection with their political activities if it is from a 'permissible donor'." It adds: "Donations of more than £500 cannot be accepted if the donor is an impermissible donor or cannot be identified."
Political gifts in Northern Ireland are kept confidential for fear of identifying donors, a legacy of the Troubles.
The DUP has said the party voluntarily published details of the donor who helped fund the DUP's participation in the European referendum campaign, a step not taken by Sinn Fein which benefits from millions of pounds worth of US money.
Meanwhile, the Information Commissioner has opened a formal investigation into the political use of private data.
Elizabeth Denham announced there would be a review into the use of data analytics for political ends amid concerns over allegations involving an analytics firm linked to a Brexit campaign.
In March, pro-Remain Labour MP Stephen Kinnock called on the Electoral Commission to investigate allegations that Leave.EU had not declared the role of the firm, Cambridge Analytica (CA), in its campaign.
An Observer investigation has found that Cambridge Analytica has close ties to Aggregate IQ, which received £32,750.73 from the DUP.
However, the DUP was affiliated to the separate Vote Leave campaign, and co-ordination between it and Leave.EU would have been illegal under United Kingdom law.
The DUP and Vote Leave have denied any such co-ordination.