Khashoggi, a prominent critic of the Saudi government, was killed inside the kingdom's consulate in the Turkish city of Istanbul by a team of Saudi agents.
The Saudi public prosecutor said it was the result of a "rogue operation" and put 11 unnamed individuals on trial.
A UN expert has concluded that it was an "extrajudicial execution".
Special Rapporteur Agnes Callamard called for Saudi Crown Prince Mohammed bin Salman to be investigated over the killing.
He has denied any involvement, but in October he said he took "full responsibility as a leader in Saudi Arabia, especially since it was committed by individuals working for the Saudi government".
The trial, which took place behind closed doors, did not meet international standards and Saudi authorities had "obstructed meaningful accountability", Human Rights Watch said.
The 59-year-old journalist, a US-based columnist for the Washington Post, was last seen entering the Saudi consulate on 2 October 2018 to obtain papers he needed to marry his fiancée Hatice Cengiz.
After listening to purported audio recordings of conversations inside the consulate made by Turkish intelligence, Ms Callamard concluded that Khashoggi was "brutally slain" that day.
Saudi Arabia's deputy public prosecutor Shalaan Shalaan told reporters in November 2018 that the murder was ordered by the head of a "negotiations team" sent to Istanbul by the Saudi deputy intelligence chief to bring Khashoggi back to the kingdom "by means of persuasion" or, if that failed, "by force".
Investigators concluded that Khashoggi was forcibly restrained after a struggle and injected with a large amount of a drug, resulting in an overdose that led to his death, Mr Shalaan said. His body was then dismembered and handed over to a local "collaborator" outside the consulate, he added. The remains have not been found.
Mr Shalaan insisted Prince Mohammed "did not have any knowledge about it".