Avon and Somerset Police were called to York Road in the Bedminster area of the city at around 1:30am on Saturday.
The force said a vehicle, with three people inside, drove through the gates and into the River Avon.
A spokeswoman said one man died while the other two people were treated for minor injuries.
The incident has been referred to police surveillance because a police vehicle was in the vicinity at the time, but, the spokeswoman said, there was no pursuit.
“We responded to an incident in the early hours of this morning, Saturday February 25, after a car drove through the riverside gates at York Road, Bedminster, and into the water,” he said.
“There were three occupants in the car. One of them, a man in his 20s, sadly passed away. Family members have been informed and our thoughts are with them during this difficult time.
“The family will receive support from a specially trained liaison officer.
“The other two occupants have received medical attention.”
The roads in the surrounding area were closed while the emergency services attended to the incident.
The spokeswoman continued: “An investigation is underway to determine how the vehicle got into the water.
“We are making a mandatory referral on this incident to the Independent Office for Police Conduct (IOPC).
“We can confirm that the mandatory IOPC referral is a result of the police being nearby at the time of the incident. There was no police pursuit.”
Police forces are required to refer certain incidents to the watchdog, for example where there has been a death or serious injury after contact with the police.
In 2018, a man and woman were rescued after the car they were traveling in crashed into bars on York Road.
And in 2019, a pickup truck driver and a passenger managed to escape after their vehicle went through the bars.