Chomsky, a professor of linguistics at the US Massachusetts Institute of Technology and a prominent critic of US foreign policy, joined scores of high-profile Australian lawyers, authors and journalists in signing the letter.
Noting the "increasingly violent rhetoric" directed towards Australian-born Assange, the besieged founder of whistleblowing website WikiLeaks, the signatories said there were "grave concerns" for his safety.
"We therefore call upon you to condemn, on behalf of the Australian Government, calls for physical harm to be inflicted upon Mr Assange, and to state publicly that you will ensure Mr Assange receives the rights and protections to which he is entitled, irrespective of whether the unlawful threats against him come from individuals or states," says the letter, published on the ABC website.
Penned by Victoria University academic Jeff Sparrow and human rights lawyer Lizzie O'Shea, the letter calls on Gillard to publicly confirm Australia's commitment to free political communication and uphold Assange's basic rights.
It also urges her to "provide assistance and advocacy to Mr Assange; and do everything in your power to ensure that any legal proceedings taken against him comply fully with the principles of law and procedural fairness."
"A statement by you to this effect should not be controversial -- it is a simple commitment to democratic principles and the rule of law," it says. Sparrow, editor of Australia's Overland literary journal, said the idea began with a few invitations but soon went viral, attracting an overwhelming response.
"Chomsky contacted us because I guess somebody had forwarded it to him," Sparrow told AFP. "I think that this sentiment and this suspicion that Mr Assange is not to receive fair treatment is something... that extends beyond Australia more generally," he added.
Also signed by Australian Greens Senator Bob Brown, army whistleblower Lance Collins and a host of Australian authors including Raimond Gaita, Christos Tsiolkas and Helen Garner, the letter said Assange's case was a "watershed" for free speech.