Nils Melzer, the UN special rapporteur on torture who visited Assange in a high-security London prison on May 9, voiced concern at fresh US criminal charges laid against Assange and reiterated a call for him not to be extradited.
Assange was too ill on Thursday to appear via video link from a British prison in a hearing on an extradition request from the United States, his lawyer Gareth Peirce told Reuters.
He is in a health ward, Presstv reported.
"Mr. Assange has been deliberately exposed, for a period of several years, to progressively severe forms of cruel, inhuman or degrading treatment or punishment, the cumulative effects of which can only be described as psychological torture," Melzer said in a statement.
The Swiss law professor, an independent expert, was accompanied on the prison visit by two medical experts specialized in examining potential victims of torture and other ill-treatment, the statement said. They spoke with Assange in private and conducted a "thorough medical assessment."
"It was obvious that Mr. Assange's health has been seriously affected by the extremely hostile and arbitrary environment he has been exposed to for many years," Melzer said.
"In addition to physical ailments, Mr. Assange showed all symptoms typical for prolonged exposure to psychological torture, including extreme stress, chronic anxiety and intense psychological trauma."
The United States has charged Assange with espionage, saying he unlawfully published the names of classified sources and conspired with and assisted ex-Army intelligence analyst Chelsea Manning in obtaining access to classified information. He faces 18 US criminal counts and decades in prison if convicted.
"My most urgent concern is that, in the United States, Mr. Assange would be exposed to a real risk of serious violations of his human rights, including his freedom of expression, his right to a fair trial and the prohibition of torture and other cruel, inhuman or degrading treatment or punishment," Melzer said.
Assange made international headlines in 2010 when WikiLeaks published a classified US military video showing a 2007 attack by Apache helicopters in Baghdad that killed a dozen people, including two Reuters news staff.
"Since (2010), there has been a relentless and unrestrained campaign of public mobbing, intimidation and defamation against Mr. Assange, not only in the United States, but also in the United Kingdom, Sweden and, more recently, Ecuador," Melzer said, pointing to the press and social media, and unnamed senior political figures and judicial magistrates involved in the case.