Announcing the decision to reopen the case in a statement, Sweden's Chief Prosecutor Marianne Ny informed that this decision was taken after a review of the case.
The reopening comes after the case was dismissed last week by Eva Finne, chief prosecutor in Stockholm, who overruled a lower-ranked prosecutor.
Finne said that there was no reason to suspect that Assange had raped a Swedish woman who had reported him to police.
On the woman's lawyer appeal, Ny has now reopened the case.
Overruling a previously finalised decision to only investigate the case as "molestation," which is not a sex offense under Swedish law, Ny said that complaint against Assange should be investigated on suspicion of "sexual coercion and sexual molestation."
WikiLeaks has a server in Sweden and Assange is seeking legal protection for the website.
Assange has denied the allegations stating that these charges were part of a smear campaign launched against him after his website published scores of classified US military documents on war in Afghanistan.