The court handed 20-year sentences to five people and three others were sentenced to between seven to 10 years, state media reported. The eight convicted were not identified.
The verdict comes after Khashoggi's sons said in May they had "pardoned" the killers, a move condemned as a "parody of justice" by a UN expert.
Jamal Khashoggi Death Case
Khashoggi, a critic of Saudi Crown Prince Mohammed bin Salman, was last seen at the Saudi consulate in Istanbul on Oct. 2, 2018, where he had gone to obtain documents for his impending wedding. His body was reportedly dismembered and removed from the building, and his remains have not been found.
The murder caused a global uproar and tarnished the reformist image of Prince Mohammed, son of King Salman and the kingdom's de facto ruler.
Khalil Jahshan, from the Arab Center in Washington, DC, noted the prosecutor's office said the announcement "closes the case forever".
At an earlier stage of the trial in December, the court sentenced five people to death and three to jail saying then that the killing was not premeditated, but carried out "at the spur of the moment. None of the defendants were named.
Some Western governments, as well as the CIA, said they believed he had ordered the killing.
Saudi officials denied he played a role, though in September 2019 the prince indicated some personal accountability, saying "it happened under my watch".
Questions remain over Saudi Crown Prince Mohammed bin Salman's (MBS) role in ordering the killing, with several western intelligence agencies alluding he had knowledge of the operation beforehand. MBS has said he had nothing to do with the murder.
The Saudi government called the assassination a "rogue operation" after repeatedly denying any involvement for weeks.