Sarawak Report, a website run by a British woman based in London, has been publishing reports and documents that allegedly include graft and mismanagement by the state investment fund 1Malaysia Development Berhad (1MDB).
The Malaysian Communications and Multimedia Commission (MCMC) said the site had breached a law under the Communications and Multimedia Act 1998, prohibiting people from using a website to provide content "that is indecent, obscene, false, menacing, or offensive in character with intent to annoy, abuse, threaten or harass any person".
"MCMC decided to block a website that could threaten the country's stability, namely Sarawak Report, for publishing contents with unproven veracity and that are under investigation, after receiving complaints from the public," the commission said in a statement on Sunday evening.
The site's editor Clare Rewcastle-Brown said it was "a blatant attempt to censor our exposures of major corruption".
"This latest blow to media freedom only brings further discredit upon the present administration, who have proven unable to counter the evidence we have presented in any other way," Rewcastle-Brown said in a statement.
Opposition lawmaker Lim Kit Siang, from the Democratic Action Party, decried the action as diminishing the government's credibility, Reuters reported.
"My advice to Najib is to allow common sense to prevail, that blocking access to Sarawak Report is not going to end his 1MDB woes but the opposite, plunging his credibility and legitimacy to even unimagined depths, which is not good for him as prime minister and for the nation as well," said Lim.
1MDB, with debts of over $11 billion, is being investigated by authorities in Malaysia for financial mismanagement and graft. The state-owned firm's advisory board is chaired by the prime minister.
The Wall Street Journal (WSJ) reported earlier this month that investigators looking into 1MDB had traced close to $700 million of deposits into personal bank accounts belonging to Najib, according to documents from the probe.
Reuters has not verified the WSJ report.
Najib has denied taking any money for personal gain and said the corruption allegations are part of a malicious campaign to force him out of office. He is weighing legal options against the WSJ.