Robertson, a barrister who has dual British and Australian nationality, has participated in some of the highest-profile freedom of speech trials in British history.
He was also among the defence team in the trial for the IRA bombing of the Brighton hotel that targeted then prime minister Margaret Thatcher in the mid-1980s.
"I do believe he is representing him," a member of staff at Robertson's Doughty Street Chambers in London told AFP.
According to his personal website, Robertson is a member of the United Nations Justice Council, having served as the first President of the Special Court in Sierra Leone.
Assange was denied bail on Tuesday and ordered to appear in court again on December 14th after he was arrested in London on a European arrest warrant linked to allegations of rape and molestation against two women in Sweden.
The 39-year-old Australian is being held in Wandsworth prison in London. His supporters claim the arrest warrant is politically motivated because WikiLeaks has angered governments around the world by releasing thousands of US diplomatic cables.