His death was confirmed by his employer, security firm IOActive, and his sister Amberleigh Jack.
Jack's death comes days before the opening of the Black Hat hacking convention in Las Vegas where he was due to give a talk on security vulnerabilities of electronic medical implants for humans, theregister.co.uk reported.
The San Francisco Medical Examiner's office said Jack, 36, died in the city Thursday, but released no details on the cause of death.
Jack was known for impressive public hacking demonstrations that made him a sought-after presenter at computer-security conferences.
In 2010, at the annual Black Hat security conference in Las Vegas, he demonstrated his ability to hack into an ATM and cause it to dispense cash, performing the hack live on stage.
Previously he had demonstrated a security flaw in an insulin pump that allowed him to hack the device from as far as 300 feet away, forcing it to dispense the hormone.
He was expected to give another medical-device presentation next week.
In an interview last week, Jack said he would show he could scan from a distance of as far as 30 feet for a certain model of pacemaker and defibrillator -- he would not identify the model -- override its software and send high-voltage shocks to the device, causing it to stop working.
His scheduled Aug. 1 presentation won't be replaced, Black Hat general manager Trey Ford said.
"We have lost a member of our family," Ford said in a statement. "Everyone would agree that the life and work of Barnaby Jack are legendary and irreplaceable."