"We found Lindsay Sandiford convincingly and legally guilty for importing narcotics… and sentenced the defendant to death," judge Amser Simanjuntak told Denpasar district court.
Sandiford was arrested at Bali's international airport in May with 4.79 kilograms (10.6 pounds) of cocaine stashed in her suitcase. Police have said she then helped mount a sting to net three Britons and an Indian on drugs charges.
The harsh sentence came as a surprise after prosecutors last month recommended Sandiford serve only 15 years in jail, saying she deserved leniency as she had admitted her crime and behaved politely in court.
However, another judge on the panel, Bagus Komang Wijaya Adi, said Tuesday "there are no mitigating circumstances" to allow for leniency in the case.
"All evidence was incriminating against the defendant," he said.
Indonesia enforces stiff penalties against drug trafficking, with penalties of death and life imprisonment.