A Moscow military court had sentenced retired military intelligence Colonel Sergei Skripal, 55, to 13 years in prison for espionage on August 9.
"The verdict on the criminal case comes into force today," the court said.
Prosecutors had demanded 15 years for Skripal, but the court had taken into account mitigating circumstances such as his admission of guilt, assistance to investigators, and poor health.
The Federal Security Service (FSB) said earlier Skripal had been recruited by the British Secret Intelligence Service, better known as MI6.
Russian media reports said Skripal had shared information on dozens of his former colleagues operating in Europe undercover, in particular their secret meeting venues, addresses, and passwords.
Skripal was reported to have received over $100,000 for his collaboration.
A major spying scandal clouded relations between Russia and the UK at the start of the year, when Moscow claimed four British agents in the Russian capital had been caught procuring information from a high-tech communication device disguised as a rock.
