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The two leaders flew in together from Damascus and were greeted at Beirut airport by Lebanese President Michel Sleiman and Prime Minister Saad Hariri, son of the slain Sunni leader, as well as other dignitaries.
They were then headed to the presidential palace for a mini-summit hastily organised to address tensions over reports of an impending indictment against members of the Shiite militant group Hizbollah for Rafik Hariri's murder.
It is the first visit to the country by Assad since Hariri's assassination soured bilateral ties and forced the pullout of Syrian troops from Lebanon after a 29-year deployment.
Damascus has consistently denied accusations that it had a hand in the killing.
Relations between the two countries have been on the mend since 2008, when diplomatic ties were established for the first time. Prime Minister Hariri has also made four trips to Syria in the past eight months.
Saudi Arabia, a staunch supporter of the Hariri family, has played a key role in the rapprochement between the Arab neighbours.
Saudi and Syrian flags were being flown throughout the Lebanese capital on Friday for the unprecedented joint visit, along with huge portraits of the king together with a welcome message.
Security was also tight, with additional army and police deployed.
The visit by the Saudi and Syrian leaders was scheduled to last only three hours and would include a lunch to be attended by some 250 officials, among them members of the unity government which includes two Hizbollah ministers.
"The whole visit is about containing the situation for the immediate future," said Sahar Atrache, a Beirut-based analyst with the International Crisis Group think-tank.
"They are here to exert influence on their internal allies to prevent a real escalation."
The Arabic-language newspaper An-Nahar, which is close to Hariri's Saudi- and Western-backed governing coalition, called the summit "historic."
"This joint visit is historic and decisive because of its timing and the consequences it can have on a mounting crisis in Lebanon related to the tribunal," it said.
Fears of renewed conflict rose last week after Hizbollah leader Hassan Nasrallah revealed that he knew the UN tribunal probing Hariri's murder was poised to indict members of his party, which is backed by Syria and Iran.
He made it clear that he would not accept such a scenario, accusing the tribunal of being politicised and part of an Israeli plot.
"The Arab leaders' visit to Lebanon is an opportunity to show Arab unity in the face of this plot which aims to destabilise Lebanon and sow sedition," Hizbollah deputy Hassan Fadlallah told AFP.