By Agence France-Presse
Quoting US Secretary of State Mike Pompeo, Mattis told a security conference in Bahrain that the United States "does not tolerate this kind of ruthless action to silence Mr Khashoggi, a journalist, through violence".
"With our collective interests in peace and unwavering respect for human rights in mind, the murder of Jamal Khashoggi in a diplomatic facility must concern us all greatly," Mattis added.
"Failure of any nation to adhere to international norms and the rule of law undermines regional stability at a time when it is needed most."
A critic of Saudi Arabia's powerful Crown Prince Mohammed bin Salman, Khashoggi was murdered after entering the Saudi consulate in Istanbul on October 2.
On Thursday, CIA Director Gina Haspel briefed US President Donald Trump on the latest developments in the investigation after a fact-finding mission to Turkey.
Pro-government Turkish media said that intelligence officers showed Haspel video images and audio tapes of Khashoggi's killing gathered from the consulate.
His murder has generated international outrage and undermined relations with Riyadh, which Washington is hoping can help counter Tehran's influence in the Middle East.
Prince Mohammed, the heir apparent to the kingdom's throne, has denounced the "repulsive" murder, denying any involvement. The Saudi leadership has pushed responsibility down the chain of command.
Trump has called the case "one of the worst cover-ups in history".
Washington moved late Tuesday to revoke the visas of several Saudis. Britain followed suit on Wednesday.
Mattis said more measures would follow.
"We will maintain our 'twin imperatives', as stated by Secretary of State Pompeo, of protecting America and holding accountable those responsible for this murder," he told the forum.
"Our secretary of state has already revoked visas and will be taking additional measures."
Turkey said on Friday it wanted Saudi Arabia to extradite 18 Saudis it has arrested over Khashoggi's murder.
President Recep Tayyip Erdogan, who has stopped short of directly blaming the Saudi government, said the 18 suspects must know who killed Khashoggi and repeated his call for the men to be tried in Turkey.
He urged Riyadh to reveal who ordered the killing and the whereabouts of Khashoggi's body.
"You need to show this body," Erdogan said.
"Unless you tell, Saudi Arabia will not be free from this suspicion."