Chief Prosecutor Marianne Ny said in a statement the decision to reopen the inquiry was taken after further review of the case.
Prosecutors last month dropped an investigation into alleged rape, but carried on investigating charges of molestation.
Mr Assange denies any wrongdoing and said last month he had been warned by Australian intelligence of plans to discredit the whistle-blower website.
WikiLeaks last month published more than 70,000 secret military files on Afghanistan in what US officials have called one of the biggest security breaches in US military history.
