Moore, who is best know for directing and producing the films Bowling for Columbine and Fahrenheit 9/11, has offered $20,000 surety for Assange, who was arrested last week over allegations of sexually assaulting two women in Sweden.
On Wednesday Assange was refused bail by the judge at the City of Westminster Magistrates' Court and has been remanded in custody until today's hearing, where he will make a renewed application for bail.
WikiLeaks' is part way through the release of more than 250,000 diplomatic cables sent from US embassies across the world, which have been published by the website and via partner media organisations, including the Guardian.
The filmmaker says he will also give "the assistance of my website, my servers, my domain names and anything else I can do to keep WikiLeaks alive and thriving". Speaking in a blog post on his website, Moore is critical of attacks from US politicians on WikiLeaks and Assange, such as senator Joe Lieberman's claims that the site has violated the US Espionage Act.
"I will not allow this injustice to continue unchallenged," he writes.
At his first bail hearing last week journalist John Pilger and film director Ken Loach were amongst those promising sureties for the whistleblowing website's editor.
In a witness statement to Westminster Magistrates Court, Moore says he supports Assange as "a pioneer of free speech, transparent government and the digital revolution in journalism".
"In September 2008, I released my first free movie on the Internet, Slacker Uprising, documenting my personal crusade to encourage more Americans to vote in presidential elections. These experiences underpinned my conviction that it is the duty of a free press to probe, and hold government and the powerful to account - and that citizens must be properly informed and have access to information in order to exercise their democratic rights," he says in the statement.
"Governments have always been discomfited by a probing press. With the hollowing out of newsrooms, in large part as a consequence of the new digital world, old media have largely abandoned the territory of investigative journalism.
"...It dishonours this court to be used in this way, holding this man without bail. Julian has made the world, and my country in particular, a safer place. His actions with WikiLeaks have put on notice those who would take us to war based on lies that any future attempts to do so will be met by the fierce bright light provided by WikiLeaks and intended to expose those who commit their war crimes. His actions will make them think twice next time - and for that we all owe him a debt of gratitude."
Moore says in the statement that absconding and breaking terms of any bail granted would harm Assange's reputation in the media and journalism industries and suggests the WikiLeaks founder would not risk this.
Moore adds that he will not be able to attend today's hearing, which is scheduled for 2pm according to Westminster Magistrates Court, but has wired money to the court.