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.
In 2013 Mr Putin reportedly proposed that Seagal be made an honorary consul of Russia in California and Arizona.
The move would have made Seagal a potential intermediary between the White House and the Kremlin, Buzzfeed reported. The US refused.
Seagal, whose grandmother was from Vladivostok in Russia's far east, has made frequent trips to Russia in recent years and visited Kamchatka and Sakhalin in September.
He said during the visit that he wanted to spend several months a year in the country, Russia's Tass news agency reported.
Seagal follows other celebrities including French actor Gerard Depardieu, US mixed martial arts fighter Jeff Monson, US boxer Roy Jones and US snowboarder Vic Wild in taking Russian citizenship.
Earlier this year Seagal was given Serbian nationality after offering to set up a martial arts school in the capital Belgrade.
The action star's movie credits include Under Siege and Flight of Fury and he has also starred in his own US reality TV show, Steven Seagal: Lawman.
US actor Steven Seagal has been given Russian citizenship, the Kremlin says.