Only the male driver escaped unhurt during the incident on the San Mateo bridge, 20 miles (32km) south-east of the city, a US police spokesman said.
The victims, said to be in their 30s, were trapped as fire engulfed the back of the car, which had stopped.
Amateur video footage showed the white car ablaze in the darkness.
No other vehicle appears to have been involved in the incident and police are still trying to determine the cause.
Severe burns
The fire was reported around 22:00 on Saturday (05:00 GMT Sunday) on the bridge which connects San Mateo and Alemada counties at Foster City.
When smoke began to pour from back of the limo, the driver quickly pulled over but fire spread rapidly.
Four women managed to get out but one had severe burns while the others suffered minor injuries including smoke inhalation, according to local broadcaster KTVU. They were taken to local hospitals.
The westbound lanes of the bridge were closed as police investigated, but one lane of traffic re-opened early on Sunday.
Details of the victims were not released as the authorities sought to notify their families.
"We have no idea right now where they were going or where they were coming from," California Highway Patrol officer Amelia Jack told KGO-TV.
Stretch limousines, usually designed to carry groups of six adults or more, are typically hired for celebrations such as school proms or pre-wedding parties.
-BBC
