In a 99-page report, Callamard said experts found it "inconceivable" that a sophisticated 15-man mission to kill Khashoggi could have happened without Prince Mohammed's knowledge.
The announcement drew anger from Khashoggi's fiancee Hatice Cengiz, who said "no one" had the right to pardon his murderers.
The sons of murdered Saudi journalist Jamal Khashoggi said Friday they "forgive" the killers of their father, an announcement analysts said effectively grants clemency to five convicted people on death row.
Khashoggi was last seen at the Saudi consulate in Istanbul on October 2, 2018, where he had gone to obtain documents for his impending wedding.
Saudi Arabia sentenced five people to death and three to jail over Khashoggi's murder last December.
Salah Khashoggi said the December verdict "has been fair to us and that justice has been achieved".More news: Canadians asked to wear non-medical face masks to protect others
The family of murdered Saudi journalist Jamal Khashoggi has forgiven those who killed their father, his son Salah wrote in a tweet on Friday. I and others will not stop until we get justice for Jamal, ' she said.
Salah, who lives in the kingdom, has denied reports of a financial settlement with the government.
"What this essentially means is that the killers will avoid capital punishment since that is a right the family [under sharia law] has to forgive", Ali Shihabi, a Saudi author and analyst close to the government said on Twitter.
The December verdict, which was lambasted by rights groups as a travesty of justice, underscored Saudi efforts to draw a line under the crisis as it seeks to reboot its worldwide image ahead of this year's G20 summit in Riyadh.
The son of murdered Washington Post columnist, Jamal Khashoggi, has released a statement "forgiving" his killers.
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He has also previously criticised "opponents and enemies" of Saudi Arabia who he said had tried to exploit his father's death to undermine the country's leadership. Three of the 11 accused were sentenced to prison terms totalling 24 years and three others were found not guilty.
Turkish and Western intelligence agencies said the order to kill him could only have come from the highest levels of the Saudi government.
However, it's not clear if the statement will hold any legal standing in the treatment of those in Saudi Arabia sentenced to death over the killing. His remains have never been found. He was confronted by Saudi agents before being kidnapped, tortured, murdered and dismembered.
The kingdom denies the crown prince had any knowledge of the operation.
The journalist, a 59-year-old critic of the prince, had written for the Washington Post and lived in the United States.
The journalist - who had gone into self-imposed exile in the U.S. in 2017 - went to the Saudi consulate in Istanbul on 2 October 2018 seeking documents to get married to fiancé Hatice Cengiz.More news: Singapore passes first Zoom-enabled death sentence to Malaysian man