The UN expert who investigated the journalist's murder called the Saudi investigation "the antithesis of justice".
Saudi Arabia sentenced five men to death and three others to prison for the murder of Jamal Khashoggi in a ruling that has been widely dismissed by experts.
The Washington Post columnist was killed in 2018 after walking into the Saudi consulate in Istanbul to obtain documents in order to marry his fiancée, Hatice Cengiz.
The murder generated international outrage and damaged Saudi Arabia's international reputation.
Saudi Arabia's public prosecutor announced the sentences on Monday but specified that a former adviser of the Saudi prince Saud al-Qahtani would not be charged.
Agnes Callamard, the UN expert who investigated the killing, called the ruling the "antithesis of justice" and a "mockery".
"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," Callamard, the UN Special Rapporteur on extrajudicial, summary or arbitrary executions tweeted.
"That is the antithesis of Justice. It is a mockery."
Kenneth Roth, the director of Human Rights Watch, tweeted that "by potentially hanging lower-level operatives while letting off scot-free those who ordered the killing" the Saudi sentences showed that there needed to be an independent probe.
Meanwhile, Salah Khashoggi, one of the journalist's two sons, said on Twitter: "We affirm our confidence in the Saudi judiciary at all levels, that we are fair and that we achieve justice."
A U.S. official told Reuters that the trial of the killers as "an important step" towards holding those responsible for his death accountable.
"We encourage Saudi Arabia to continue with a fair and transparent judicial process," the official, who declined to be named, said.
A UN report said that evidence suggests the execution would have required "significant government coordination, resources and finances" and found that Khashoggi was the victim of a "premeditated extrajudicial execution".
"Every expert consulted finds it inconceivable that an operation of this scale could be implemented without the Crown Prince being aware at a minimum, that some sort of mission of a criminal nature, directed at Mr Khashoggi, was being launched," the report by UN special rapporteur Callamard said.
In Khashoggi's last published column for the Washington Post, he wrote about press freedom in the Arab World.
"A state-run narrative dominates the public psyche, and while many do not believe it, a large majority of the population falls victim to this false narrative. Sadly, this situation is unlikely to change," he said.
Turkey said the judgement was an insult to the rest of the world.
“To claim that a handful of intelligence operatives committed this murder is to mock the world’s intelligence — to say the least,” the country’s communications director Fahrettin Altun said on Twitter.
“Turkey will continue its efforts to shed light on this incident. Those who dispatched a death squad to Istanbul on a private jet, signed Khashoggi’s death warrant, disappeared the slain journalist’s body and sought to sweep this murder under the rug have been granted immunity.”