The Swedish hacker is serving 50 weeks for violating his bail conditions after seven years holed up at the Ecuadorian Embassy in London.
Assange, 47, faces extradition to the US over his Wikileaks activities.
He met Nils Melzer who usually reports on prison torture in corrupt states at HMP Belmarsh, South East London on May 10.
He was also visited by human rights expert and privacy campaigner Joseph Cannataci on April 25.