United Kingdom District Judge Vanessa Baraitser delivered the decision against the US request on Monday, at the Old Bailey court in London.
She said Assange was “a depressed and sometimes despairing man” who had the “intellect and determination” to circumvent any suicide prevention measures taken by prison authorities.
If detained in the US, Assange “faces the bleak prospect of severely restrictive detention conditions designed to remove physical contact and reduce social interaction and contact with the outside world to a bare minimum”, she said.
“He faces these prospects as someone with a diagnosis of clinical depression and persistent thoughts of suicide,” she said in her ruling.
“I am satisfied that the risk that Mr Assange will commit suicide is a substantial one.”