The United States and Saudi Arabia have a decades-old relationship based on the exchange of American security for Saudi oil.
But ties between Riyadh and Washington became increasingly frayed during the administration of president Barack Obama.
Saudi leaders felt Obama was reluctant to get involved in the civil war in Syria and was tilting toward Riyadh's regional rival Iran.
The Sunni Muslim kingdom "felt marginalised" during international negotiations on a nuclear accord with Shiite Iran, an American defence official said.
That deal, signed in July 2015 by the Obama administration, saw the lifting of international sanctions in exchange for guarantees that Tehran will not pursue a nuclear weapons capability.
Mattis arrived in Riyadh yesterday afternoon, wishing to "reinvigorate" ties by listening to Saudi leaders and learning "what are their priorities", the official said.
The retired four-star Marine general will meet King Salman, Crown Prince Mohammed bin Nayef, and Salman's son Deputy Crown Prince Mohammed bin Salman, who holds the defence portfolio.
The deputy crown prince last month met President Donald Trump in Washington.
Saudi leaders worry about Iran interfering in Arab countries by using Shiite communities to advance their pawns, as in Bahrain, Lebanon and Yemen.
Bordering Saudi Arabia, Yemen has been torn for more than two years by a civil war between Iran-backed Huthi rebels, their allies, and pro-government forces aided by a Saudi-led military coalition that receives some military support from the US.
The Saudis have found a more favourable ear in Washington under Trump, who denounces Iran's "harmful influence" in the Middle East.
Mattis has called Iran "the single biggest state sponsor of terrorism in the world".
In February, Trump imposed new sanctions on Tehran after a ballistic missile test launch, and in response to its support for Yemen's rebels.
The US military is watching Huthi activities along the strategic Bab al-Mandab strait connecting the Red Sea with the Indian Ocean.
Rebels in late January attacked a Saudi warship in the Red Sea, and they are also believed to have fired missiles towards US warships in the area.
The United States accuses the rebels of deploying coastal defence missiles and other weapons which threaten free navigation in the waterway which is vital to global trade.
"I am extraordinarily concerned about another contested maritime chokepoint in the region," US Central Command chief General Joseph Votel told the House Armed Services Committee in March.
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