His co-defendant, the activist and anti-fascist Alexander Kolchenko, was sentenced to 10 years.
The judge found Sentsov guilty of setting up a terrorist organisation and committing two terrorist acts.
When asked by the presiding judge Sergei Mikhailyuk whether they understood the verdicts, Sentsov and Kolchenko responded by defiantly singing the Ukrainian national anthem Ukraine Has Not Yet Died.
Sentsov, best known for his 2011 film Gamer, was arrested in May 2014 during a protest against Russia’s annexation of the Ukrainian peninsula two months earlier.
The 39-year-old director was accused of plotting to blow up a monument to Lenin in Crimea and set fire to the Crimean offices of pro-Moscow political organisations.
The Ukrainian government said he is being punished for being a Crimea-based pro-Ukrainian activist. Russia denies claims he is a political prisoner.