Hatice Cengiz made the shock comments during a memorial to her late-partner in London.
Ms Cengiz emotionally said: “President Trump should help reveal the truth and ensure justice is served.
“He should not pave the way for a cover-up of my fiancee’s murder.
“Let’s not let money taint our conscience and compromise our values.”
Mr Khashoggi, a prominent dissident, vanished after entering the Saudi Arabian consulate in Istanbul on October 2.
He had been instructed to visit the facility to pick up marriage documents.
Ms Cengiz, who was waiting outside, raised the alarm after her fiancé didn’t reappear after ten hours.
After initially claiming he left the back of the building Saudi authorities have admitted Mr Khashoggi died in the consulate, though they have been unable to produce a body.
Turkish authorities insist he was killed in a premeditated murder, whilst the Saudi government says he was killed by accident during a brawl.
Ms Cengiz has repeatedly stated her fiancee was murdered.
At the London memorial she said: “If only I knew what would happen, I would have entered the consulate myself.
“If only I knew that would be the last time i would see my Jamal, I would have stood in front of that murderous team myself.
“We never imagined such a level of barbarity, cruelty and evil could be waiting for Jamal.”
The US has revoked the visas of 21 Saudi officials they link to Mr Khashoggi’s death.
President Trump has accused Saudi authorities of carrying out the “worst cover-up ever” over the incident.
However he has pointedly refused to halt weapons sales to the Kingdom saying: “They’re spending $110billion on military equipment and on things that create jobs for this country.
“I don’t like the idea of stopping an investment of $110billion into the United States.”