President Putin signed a decree to make the award to the martial arts expert, who starred in Hollywood action thrillers during the 1990s.
Seagal is friends with Mr Putin, who he has described as “one of the greatest world leaders”.
Mr Putin’s spokesman said Seagal was known for his warm feelings towards Russia and his status as a famous actor was a reason to make him a citizen.
Seagal made headlines when he described Russia’s actions in Crimea, which it annexed in 2014, as “very reasonable”. A keen guitarist, he played a concert there the same year.