In talks with the six nations of the Gulf Cooperation Council -- Saudi Arabia, Bahrain, Kuwait, Qatar, Oman and the United Arab Emirates -- Kerry said agreement was reached on how to begin peace talks aimed at ending Syria's bloody civil war.
"The relationship between the United States and the GCC nations is one that is built on mutual interest, on mutual defense, and I think there is no doubt whatsoever in the minds of the countries that make up the GCC that the United States will stand with them against any external threat," Kerry said after the meeting.
He then met with Saudi King Salman and Deputy Crown Prince Mohammed bin Salman.
Kerry's visit was aimed at soothing nervous Gulf allies who are concerned about a rapprochement between the U.S. and Iran following last week's implementation of the Iran nuclear deal that lifted crippling sanctions on Iran in exchange for curbs on its nuclear capabilities.
Saudi Arabia and its Gulf neighbors have complained of a lack of U.S. engagement in the face of what they see as Iranian meddling in the region.
But following the talks, Kerry and Saudi Foreign Minister Adel al-Jubeir projected a united front about the need to confront Iran's destabilizing behavior in the region and on the need to start peace talks to end the Syrian conflict.
"The United States remains concerned about some of the activities that Iran is engaged in in other countries," Kerry said, pointing to continued U.S. sanctions over Iran's support for terrorist groups like Hezbollah, its human rights record and its development of ballistic missiles.
Despite the improved relations between the two countries, al-Jubeir said he didn't foresee a new alliance between Washington and Tehran.
He pointed to the work that the U.S. and its Gulf allies were doing together to bring peace and security to Middle East. Much of that, he said, involved pushing back against Iran's "hostile and aggressive stance" in the region.
"No, I don't see a coming together of the United States and Iran. Iran remains the world's chief sponsor of terrorism," al-Jubeir said. "Overall, I think the United States is very aware of the danger of Iran's mischief and nefarious activities can do in the region ... I don't believe the United States is under any illusion as to what type of government Iran is."
With the nuclear deal finished, Kerry has turned his attention to finding a political solution to end the civil war in Syria. But his hopes that Iran now become a more willing partner have been somewhat dampened because of a widening diplomatic rift between Tehran and Riyadh.
Iran has been a longtime supporter of Syrian President Bashar al-Assad, while Saudi Arabia has provided the rebel factions fighting Assad with financial assistance and weapons.
After months of prodding, Kerry was able to get the historic rivals to sit at the same table last month for the first time since the war began five years ago to discuss a road map to end the conflict.
But Riyadh's execution earlier this month of a prominent Shiite cleric and Iranian recriminations have brought the tensions to a new low. They spiked further when, after a mob attacked the Saudi embassy in Tehran in protest, the kingdom and several of its allies severed diplomatic and some commercial ties with Iran.
A senior state department official said Friday the U.S. supports Saudi in the latest row, but hoped its longtime partner would restore diplomatic ties with Tehran after Iran's Supreme Leader Ayatollah Ali Khamenei apologized this week for the storming of the embassy.
Kerry has expressed concern about the feud. Before coming to Riyadh, he said in Switzerland he hoped Syria could be a focal point for reconciliation between the two longtime rivals, stressing that all countries have an interest in defeating ISIS and al-Qaeda affiliates like the al-Nusra Front, which are exploiting the political chaos and violence in the country.
Both countries have pledged not to let their rivalry affect Syria peace efforts, and U.S. officials say there are no signs the hostility has negatively affected the diplomacy.
But differences between the two sides about which Syrian groups should represent the opposition at the bargaining table have threatened to delay the start of U.N.-led talks set to begin Monday in Geneva.
Kerry said an "understanding" had been reached Saturday on how to begin the Syria talks and expressed confidence they would start next week as planned.
"We are quite confident that there is a way to invite the various interested stakeholders that provides for cohesion and the ability to make the process move forward," Kerry said, adding that would powers would meet immediately after the first round to address any issues that might arise in Geneva.
Later Saturday, Kerry was due to have talks with Riad Hijab, chair of the Syrian opposition's High Negotiations Committee, which was formed in Saudi Arabia last month.