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 authorities said it was the result of a "rogue operation" and put 11 unnamed individuals on trial.
A UN expert said the trial represented "the antithesis of justice".
"Bottom line: the hit-men are guilty, sentenced to death. The masterminds not only walk free. They have barely been touched by the investigation and the trial," Special Rapporteur Agnes Callamard wrote on Twitter.