Thursday's rains were at least 15 times the average annual rainfall for Jeddah, according to some estimates.
Unusual scenes were posted on local media including a video of a boat purportedly belonging to the Civil Defence, transporting people who became marooned when roads were submerged.
“I tried going to work but saw loads of accidents and you can see the emergency cars on the streets. People just left their cars. which were half flooded with water, and started walking, the streets are flooded,” said Ali Harbi, a Saudi resident in Jeddah.
Some flights at King Abdulaziz Airport were delayed.
Jeddah's health authorities denied rumours that the rainfall had caused disruption to medical services.
"Water seeped into some of the facilities in King Abdulaziz city in Jeddah and the situation was dealt with according to the emergency plan in place," the Ministry of National Guard Health Affairs said in a statement.
"Medical care services were unaffected and no injuries were reported."
More rainfall is expected on Friday as Jeddah experiences the "highest ever levels of rainfall since 2009", National Centre for Meteorology spokesman Hussein Al Qahtani said.