Warmbier, 21, was arrested for trying to steal a propaganda sign from a hotel while visiting North Korea in January.
He later appeared on state TV apparently confessing and saying a church group had asked him to bring back a "trophy" from his trip.
North Korea sometimes uses the detention of foreigners as a means of exerting pressure on its adversaries.
The BBC's Stephen Evans in South Korea says the 15-year sentence is high compared to those given to foreigners in the past.
This could be due to the particularly high tensions at the moment between North Korea and the US, he says.
'Worst mistake'
North Korean state news agency KCNA said Warmbier was convicted under an article of the criminal code relating to subversion. The verdict was handed down by the Supreme Court.