The ruling was released Thursday at noon, and no justices wrote a dissenting opinion, The Jakarta Post reported.
Sandiford, 57, was arrested in May 2012 after flying to Indonesia from Bangkok.
She was accused of participating in a drug ring with three other Britons, but at her trial, Sandiford said she was forced to smuggle drugs because her co-defendants threatened to kill her son.
She was later convicted of smuggling 10.3 pounds of cocaine into Bali.
Although prosecutors sought 15 years in prison, the Denpasar District Court sentenced her to death.
The Bali High Court then rejected her appeal against the death penalty.
Sandiford has received international support from human rights and legal organizations.
One of the other Britons, Julian Ponder, 43, of Brighton, was imprisoned for six years in January after being cleared of smuggling but convicted of possessing of cocaine.
The two other Britons also were found innocent of trafficking, with one sentenced to four years in prison for drug possession and the other to one year in prison for failure to report a crime.