The Wall Street Journal reported today that hundreds of millions of dollars more than previously identified was transferred into Najib's accounts.
The 1MDB investment fund said in a statement said that it has never paid money into Najib's personal accounts.
It accused the Wall Street Journal of not offering evidence to back its claims and relying on anonymous sources.
Meanwhile, prime minister Najib rejected the report.
A statement issued by Najib's office did not comment on the larger amount reported by the newspaper, saying only that "the funds received were a donation from Saudi Arabia."
It accused the newspaper of "relying solely on anonymous sources that may not even exist" and of "choosing to omit key known facts."
"This is unethical and against accepted journalistic practice," the statement said.
Former Malaysian Prime Minister Mahathir Mohamad told reporters earlier today that there must be some truth to the Wall Street Journal's allegations because Najib has not taken any legal action against the newspaper.
Mahathir, a fierce critic of Najib, quit the ruling party yesterday, saying it has been hijacked by Najib to protect his own interests.
