Josep Borrell, High representative of the European Union for Foreign Affairs, called on Riyadh on Tuesday to bring to justice those "responsible and involved" in the killing in Istanbul in 2018 after a ruling on the case on Monday, Efe news reported.
He reiterated the need to fully guarantee accountability and prosecution of those responsible and implicated in the murder on 2 October at the Saudi Consulate in Istanbul, a spokesman for the high representative said in a statement.
Borrell added that the practice has to be based on the principles of transparency, respect for legal procedures and due process.
His statement came after Saudi Arabia's public prosecution on Monday sentenced five people to death over the killing.
Shalan al-Shalan, spokesman for the public prosecutor, said in a press conference that three other convicts received a total of 24 years in jail for covering up the crime.
He added that 10 others were released due to lack of evidence.
Before the verdict, which can still be appealed, Borrell made it clear that the EU was unequivocally opposed to the use of capital punishment "without exception".
He described the death sentence as a cruel and inhuman punishment that fails to deter and represents an unacceptable denial of human dignity and integrity.
Khashoggi´s son praised the sentence, showing confidence in the Saudi judiciary system.
"Fairness of the judiciary is based on two principles, fairness and speedy litigation, so there is neither injustice nor procrastination," Salah tweeted.
But a number of authorities, including the Turkish government criticized the verdict.
Similarly United Nations' special rapporteur on extrajudicial killings Agnes Callamard, who investigated Khashoggi's murder, called the trial a "mockery".
Reporters Without Borders also criticized the sentence and obscurity of the trial.
Khashoggi, a staunch critic of the Saudi crown prince, was killed on 2 October after he entered the Saudi consulate in Istanbul to pick up paperwork needed for his planned marriage to his Turkish fiancée.
Saudi officials initially denied Khashoggi's death at the consulate but later acknowledged he had been killed when a fight broke out.
Saudi Arabia has said Khashoggi was killed in a "rogue operation" and put 11 defendants, all of whom are Saudi nationals, on trial earlier this year.
Although the UN and the US Senate held the Crown Prince responsible for the crime, the kingdom's strongman has not been prosecuted.
Khashoggi, 59, had been a columnist for the Washington Post since 2017 after he left Saudi Arabia and was openly critical of the country's monarchy.
The Post has condemned Khashoggi's killing, with its editorial page editor Fred Hiatt describing it as "a monstrous and unfathomable act" in October.