The meeting, bringing together armed and political factions, aims to create a common position from which to negotiate with the government.
The powerful Jaish al-Islam rebel group said it would attend the Saudi talks.
World powers meeting in Vienna want formal talks to begin between the Syrian government and opposition by January.
Division among the different Syrian opposition factions has hampered diplomacy, with a previous Turkey-based political opposition seen as out of touch with fighters on the ground.
However Iran, which is taking part in the Vienna talks and supports Syria's President Bashar al-Assad, has warned that the meeting in Riyadh could cause the talks to fail.
Jaish al-Islam is a prominent armed opposition faction near Damascus and controls most of the Eastern Ghouta suburb.
But the group's commander Zahran Alloush will not attend, reportedly because his forces lost control of a road he was planning to use to leave Damascus.
A dozen Free Syrian Army (FSA) rebel groups will also attend, including groups vetted by the United States that have received foreign military aid.
"Saudi is a pivotal state in the region and for it to take this step - to host a conference of the Syrian opposition factions - certainly something real will result from it," the head of one of the FSA groups told Reuters.
The Ahrar al-Sham group - which has fought alongside al-Qaeda-linked Nusra Front - has also been invited but has not yet said if it will attend.
However, Nusra Front and so-called Islamic State will not attend.
A Syrian Kurdish group that controls large parts of northern Syria and has fought against IS with US aid also said it had not been invited.
One of the Kurdish-run enclaves in Syria said the Saudi talks were "doomed to fail" without Kurdish involvement.
Syria's fragmented opposition is due to meet in Riyadh later in an attempt to unify ahead of possible peace talks.