The latest charges accuse him of receiving and unlawfully publishing the names of classified sources.
He was previously charged last month with one count of conspiring with ex-intelligence analyst Chelsea Manning to gain access to the Pentagon network.
Assange is serving a jail sentence in the UK for jumping bail.
It was while he was on bail facing sexual assault allegations in Sweden that he sought asylum in the Ecuadorean Embassy in London in 2012. He has always denied the allegations against him.
Ecuador abruptly withdrew his asylum last month. The 47-year-old was arrested on 11 April and later jailed for 50 weeks for skipping bail.