The party controls a faction in the lower house of the Russian parliament and plans to take part in a parliamentary election in September.
Seagal, a U.S.-born martial artist, is best known for producing and starring in action movies, while Putin, who granted him the citizenship, is a fan of martial arts.
At Saturday's ceremony, Seagal posed for pictures next to one of the party leaders, Russian writer Zakhar Prilepin, who enrolled in an army of pro-Russian separatists in eastern Ukraine and admitted to commanding a combat unit in a conflict which has killed 14,000 people in seven years.
In 2018, Russia tasked Seagal with improving humanitarian ties with the United States at a time when relations between the two countries have deteriorated to their worst level since the Cold War.
As a Russian representative, Seagal visited Venezuela earlier in May and presented a samurai sword to Venezuelan President Nicolas Maduro.