Known as "Mad Dog," the 66-year retired general held talks with members of the Saudi royal family during his second day in the capital Riyadh on Wednesday to relay US President Donald Trump's praise for the kingdom.
"It's good to be back," he told Saudi King Salman bin Abdulaziz Al Saud at al-Yamama Palace.
Mattis, who arrived in Saudi Arabia on Tuesday, also met with the Saudi defense minister, Deputy Crown Prince Mohammed bin Salman, after the king.
"It is in our interest to see a strong Saudi Arabia," he told the Saudi military chief, referencing the country's "military security services and secret services."
He further asserted that his presence in Saudi Arabia could pave the way for a Trump visit to the monarchy, a long-time US ally in the Middle East.
"What we can do here today could actually open the door possibly to bringing our president to Saudi Arabia," Mattis said.
Mattis also repeated the Trump administration’s claims that Tehran aims to “destabilize” the region.
This is while Washington has itself been a staunch supporter of “Takfiri-Wahhabi terrorists,” according to Iranian Defense Minister Brigadier General Hossein Dehqan.
Mattis further asserted that the United States aimed to continue its military presence in the Middle East.
"And now that we have the blessing of our leadership, that it's important we actually do something with it. We actually do something as we reinforce Saudi Arabia's resistance to Iran's mischief, and make you more effective with your military, as we work together as partners. We are not leaving this region," said the Pentagon chief.
Meanwhile in the US capital, Secretary of State Rex Tillerson praised Riyadh for remaining a US ally this long.
"I am pleased to be here today to reaffirm the very strong partnership that exists between the United States and the Kingdom of Saudi Arabia. We have a long relationship with Saudi Arabia that extends over 80 years and our support for a strong and steady partner on economic cooperation remains as firm as ever."
An unnamed US defense official speaking to Reuters said Mattis was in Saudi to see “what are their priorities."
The Trump administration’s anti-Iran rhetoric in part reflects the New York businessman’s long-held grudge for former President Barack Obama, who threw his support behind talks between Iran and the world powers, which finally yielded a nuclear deal, dubbed the Joint Comprehensive Plan of Action (JCPOA) in 2015.
The successful outcome of Iran talks then made Saudi Arabia feel “marginalized," according to the US official.