The 16-year-old was a passenger in a car being pursued by police in Royston, Hertfordshire early on Monday.
The Independent Police Complaints Commission (IPCC) said at about 1am, two police sergeants patrolling in a marked police car saw two cars perform u-turns in front of them on the A505.
A spokesman said: “Initial accounts from the officers say that one of the cars, a Citroen Saxo, then sped away, so they responded by using their emergency lights to signal for the car to pull over.
“The pursuit carried onto Melbourn Street in Royston town centre where the Saxo overturned.
“A 16-year-old passenger in the Saxo was pronounced dead at the scene and another man was taken to hospital with minor injuries.”
The IPCC said the incident was referred to them by Hertfordshire Police, as the Saxo was being pursued by police before the crash.
The spokesman said investigators were deployed to the scene to start collecting evidence and an independent investigation has begun.
The watchdog said Hertfordshire Police are also investigating, and three teenagers have been arrested and bailed as part of their inquiries.