The pirates, who boarded the tanker on November 15, had demanded at least $15m to release the vessel.
Andrew Mwangura, of the maritime group the East African Seafarers Assistance, confirmed that the vessel was on the move.
"The last batch of gunmen have disembarked from the Sirius Star. She is now steaming out to safe waters," he said.
Countries including the US, Russia and India have sent naval ships to patrol waters around the Horn of Africa as acts of piracy increasingly threaten the safe passage of maritime cargo through the Gulf Aden towards the Suez Canal.
Somali pirates are holding 17 ships and more than 250 crew members hostage, and have carried out more than 100 attacks in the last year.