"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 a statement.
"Most importantly, 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," he said.
Melzer, who was accompanied on the visit by two medical experts specialised in examining potential torture victims, said there was "overwhelming" evidence that Assange had been "deliberately exposed, for a period of several years, to progressively severe forms of cruel, inhuman or degrading treatment or punishment.
"The cumulative effects," he said, "can only be described as psychological torture." Assange, 47, sought refuge at Ecuador's embassy in 2012 to avoid extradition to Sweden, where he faces accusations of sexual assault.
He is currently serving a separate 50-week sentence at London's Belmarsh prison for skipping bail.
Assange now also has an extradition request against him from the US, which has charged him with violating the US Espionage Act by publishing a huge cache of military and diplomatic files in 2010, rejecting his claim that he is a journalist.
Melzer warned that extraditing Assange to the US would expose him "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." "I am particularly alarmed at the recent announcement by the US Department of Justice of 17 new charges against Mr. Assange under the Espionage Act, which currently carry up to 175 years in prison," he said.
"This may well result in a life sentence without parole, or possibly even the death penalty, if further charges were to be added in the future," he warned.
Melzer voiced outrage at the "sustained and concerted effort by several states" over the past nine years to secure Assange's extradition to the US. 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," he said.
"In 20 years of work with victims of war, violence and political persecution I have never seen a group of democratic states ganging up to deliberately isolate, demonise and abuse a single individual for such a long time and with so little regard for human dignity and the rule of law," he said.