The Malaysian Anti-Corruption Commission said in a written statement the money was a "donor's contribution" but did not reveal who the donor was, nor its purpose.
The commission said the money was not from 1Malaysia Development Berhad (1MDB), a state investment fund headed by Mr Najib that is $US11 billion in debt and the subject of several investigations in Kuala Lumpur.
Earlier the fund denied transferring money to the Prime Minister.
Mr Najib has threatened to sure the Wall Street Journal over a July 2 report that claimed Malaysian investigators had traced almost $US700 million of deposits into what investigators believed were Mr Najib's personal bank accounts after the movement of cash among agencies, banks and companies linked to the fund.
The newspaper reported that the original source of the money was unclear.
Mr Najib, who has denied taking any money for personal gain, made no comment on the commission's statement.
Allegations swirling around 1MDB are the biggest threat to Mr Najib's credibility since he took office in 2009 and have bitterly divided his long-ruling United Malays National Organisation (UMNO).
The Prime Minister has refused to resign pending the outcome of the investigations, saying the allegations are part of a malicious campaign to force him from office.
Last week Mr Najib dismissed four ministers and his deputy prime minister who had urged him to respond to the allegations involving the fund. He reshuffled his cabinet in a bid to shore up support.
At the same time, authorities in Kuala Lumpur have widened a crackdown on Mr Najib's critics, forcing the shutdown of newspapers and arresting several dozen people.
Several UMNO officials suggested recently that, as party president, Mr Najib is authorised to handle political donations.
But former prime minister Mahathir Mohamad, who has led calls for Mr Najib's resignation, wrote in his blog on Monday that hundreds of millions of dollars seemed far more than necessary for any campaign expenses.
Dr Mahathir said when he was prime minister he never had election funds deposited into his personal accounts.
The anti-corruption body said it has referred its findings to the Attorney-General and urged people to be patient while investigations continue.
As I've said many times before, their dirty deeds are one thing but the excuses these politicians use when caught red-handed are always just ridiculous. They show a contempt for the citizen who is expected to believe their lies. Just once I want to see one of them say "you got me, I'm guilty, I did it" - will be waiting a while I expect.
Posted by Arun Muruga on August 4, 2015 12:30
Editor Comment:
We are hoping ''donors'' will not overlook Phuketwan's bank account. We would cheerfully find a good use for $700 million. Brown envelopes stuff with bills are also something we could use.