In a phone call with US President Donald Trump on Monday, the king denied "any knowledge of whatever may have happened 'to our Saudi Arabian citizen'", the US leader said in a tweet.
Trump added he was "immediately" sending Secretary of State Mike Pompeo to Riyadh "to meet with the king" for talks on the crisis.
The disappearance of Khashoggi, last seen entering the Saudi consulate in Istanbul on October 2, has whipped up international concern, with new Crown Prince Mohammed bin Salman increasingly the focus of sharp criticism.
Turkish officials have said they believe Khashoggi -- a Washington Post contributor who was critical of Prince Mohammed's policies -- was killed inside the mission, and lurid claims have even been leaked that he was tortured and even dismembered.
After an initial silence, Riyadh said Khashoggi had left the consulate safely and then at the weekend denied as "baseless" claims that orders were given to have him killed.
As western companies began to distance themselves from the oil-rich Gulf nation which is seeking outside investment to diversify its economy, the king also Sunday discussed the scandal with Turkish President Recep Tayyip Erdogan.
He reiterated Riyadh's "solid" relations with Ankara, according to the state-run Saudi Press Agency.
It is not the first time that King Salman has sought to ease tensions surrounding the 33-year-old Prince Mohammed, who has been cultivated by the US but whose reformist credentials are being quickly shredded by the mystery surrounding Khashoggi's fate.