Video report by ITV News Correspondent Emma Murphy
The writer was murdered in the Saudi consulate in Istanbul last year by a team of Saudi agents.
Saudi Arabia’s state-run Al-Ekhbariya TV channel reported three others were given prison sentences, which can be appealed.
Saudi crown prince Mohammed bin Salman drew international condemnation over the killing because several Saudi agents involved worked directly for him.
The kingdom has carried out the trials of the accused in near total secrecy.
State TV also reported the Saudi attorney general's investigation showed that the crown prince's former top adviser, Saud al-Qahtani, had no proven involvement in the killing.
Al-Qahtani, however, has been sanctioned by the United States for his alleged role in the operation.
ITV News Correspondent Emma Murphy on the reaction in Washington to the ruling
The court also ruled the Saudi consul-general in Istanbul at the time, Mohammed al-Otaibi, was not guilty.
He was released from prison after the verdicts were announced, according to state TV.
After holding nine sessions, the trial concluded that there was no previous intent by those found guilty to murder, according to state TV.
The the trials of the accused were carried out in near total secrecy, though a handful of diplomats, including from Turkey, as well as members of Khashoggi's family were allowed to attend the sessions.
The killing had shocked the world and drawn condemnation from the international community, including the United Nations.
Khashoggi had walked into his country consulate in Istanbul on that morning in October 2018 to collect documents that would allow him to wed his Turkish fiance, Hatice Cengiz, who waited for him outside and he never walk out.
Agnes Callamard, a UN special rapporteur who authored an inquiry into Khashoggi's killing, later said the search for justice must not be left to the Saudi judicial system, which is so vulnerable to political interference.
She told ITV News the day of the verdict is "a sad day" for the journalist's family. She added it was "a slap in the face" for him and the people who have fought for freedom of expression.
US President Donald Trump condemned the killing, and his administration sanctioned 17 Saudis suspected of being involved, though not the crown prince.
Trump, however, has steadfastly resisted calls by members of his own party for a tougher response and has defended maintaining good relations with Saudi Arabia, framing its importance as a major buyer of U.S. military equipment and weapons and saying this creates American jobs.
Meanwhile, numerous critics of the Saudi crown prince remained imprisoned and face trial for their acts of dissent.