Hosted by the Global Investigative Journalism Network (GIJN), the petition currently has 113 signatures from journalists in countries including the UK, US, Denmark, South Africa and South Korea.
The full list of signatories will be made public when more have joined the petition, but representatives from the the Forum for African Investigative Reporters and the Centre for Investigative Journalism in the UK have already put their names to the campaign, alongside other prominent investigative journalists and writers.
The petition's statement on the GIJN website says the group is particularly concerned with the reaction to WikiLeaks following the release last month of 400,000 previously classified US military documents relating to the Iraq war.
"We, journalists and journalist organisations from many countries, express our support for Mr Assange and Wikileaks. We believe that Mr Assange has made an outstanding contribution to transparency and accountability on the Afghanistan and Iraq wars, subjects where transparency and accountability has been severely restricted by government secrecy and media control. He is being attacked for releasing information that should never have been withheld from the public," says the petition.
"We believe Wikileaks had the right to post confidential military documents because it was in the interest of the public to know what was happening. The documents show evidence that the US Government has misled the public about activities in Iraq and Afghanistan and that war crimes may have been committed."
The group concedes that criticism of WikiLeaks for not vetting its Afghanistan war logs, for example removing names of informers, is legitimate, but says there is no evidence that this lack of redaction has resulted in injury or death of those names.
"We note that Wikileaks learned from that mistake and has been much more careful with the Iraq documents. Overall, Wikileaks' factual reporting of numerous undisputed abuses and crimes is of far greater significance than the widely criticized mistakes over inadequate redacting," it says.
WikiLeaks has been "an extraordinary resource for journalists around the world" since its launch, the group says, adding that personal threats to its founder Julian Assange set a "terrible precedent... contrary to open government".
"Although it is not part of the media, and does not purport to be, its mission of informing the public and reducing unjustified secrecy complements and assists our work. As grateful beneficiaries of Wikileaks and Mr Assange's work, we stand in support of them at this time," says the site.