EU foreign affairs chief Federica Mogherini was due in Saudi Arabia on Monday to explain the agreement she helped broker on Iran's nuclear programme, and to push for an end to Yemen's war.
Mogherini is to hold talks with Foreign Minister Adel al-Jubeir in the latest visit by a top Western official aimed at easing Saudi concerns over the deal with its regional rival.
On Tuesday she flies to Iran to discuss implementation of the July 14 Vienna agreement that seeks to curb any Iranian attempt to get an atomic bomb.
The European Union played a leading role in years of talks between Britain, China, France, Russia, the United States, Germany and Iran. US Defence Secretary Ashton Carter visited Saudi Arabia to discuss the deal last week.
Mogherini has hailed the agreement as a "sign of hope for the entire world".
"She believes it's a good deal and should be welcomed," a European diplomatic source told AFP ahead of her Riyadh visit.
The accord requires Iran to curb its nuclear capabilities including the number of uranium centrifuges.
International monitors will supervise the process, and in exchange an embargo that has crippled Iran's economy will be eased.
The deal would see Iran's oil exports gradually resume and billions of dollars in frozen assets unblocked.
Last week, Jubeir said the agreement appeared to have effective safeguards, including an inspection mechanism as well as a provision to reinstate sanctions if world powers feel Iran has not met its commitments.
But he said Tehran's support for regional "terrorism" remained a concern.
Riyadh and its fellow Sunni-dominated neighbours accuse their Shia regional rival of meddling in Bahrain, Iraq, Syria, Lebanon and Yemen.
The diplomatic source said there was "a need for a political solution" in Yemen, another subject to be raised during Mogherini's visit.
"Maybe the latest developments on the ground will make this easier," the source said.
Iran-backed Houthi rebels, aided by forces loyal to former president Ali Abdullah Saleh, advanced from their traditional northern stronghold in Yemen last year.
They seized territory and moved on the southern city of Aden where internationally supported President Abedrabbo Mansour Hadi took refuge.
He fled to Saudi Arabia, which assembled an Arab coalition that began bombing the Houthis.
Anti-rebel fighters on the ground last week regained control of much of Aden.
The coalition unilaterally announced a five-day truce that began at midnight (Sunday 2100 GMT) so aid can reach a country facing what the United Nations has described as an "unfolding humanitarian catastrophe".
The diplomatic source said Mogherini would also discuss Syria and Iraq, where Islamic State group jihadists have seized territory and carried out widespread atrocities.
European nations, Saudi Arabia and other Arab states are part of a US-led military coalition against IS, a group which has inspired attacks in Europe as well as in the kingdom.