The move is the latest effort to preserve the country's "traditional" values and comes as Russian officials claim to be "at war with the collective West."
A conviction for participating in an "extremist" organization carries a maximum sentence of six years in jail.
Conservative leaders praised the "extremist" designation, calling it "historic" and "moral." However, liberal leaders and activists are outraged and said it opens the door to criminalizing any LGBT advocacy as the LGBT movement is vaguely defined.
Conservative lawmaker Pyotr Tolstoy, who called "LGBT" a "well-organized project to undermine traditional societies from within," said the Supreme Court's decision "is a historic event because our country has encroached on the most 'sacred' thing that exists in the liberal world," according to The Moscow Times.
The Orthodox Church also welcomed the move.
Vakhtang Kipshidze, an official for the Moscow Patriarchate, said: "It's a form of moral self-defense of society."
Alexei Sergeyev, a St. Petersburg-based civil rights and LGBT activist, told the paper that he might have to leave Russia due to the new threat of imprisonment.
"There are still some LGBT rights activists here in Russia. But they might well be the last ones," he said. "As I always told myself, 'I can handle the fines, but if there is a threat of imprisonment, I will leave'."
In 2013, Russian President Vladimir Putin signed a bill into law that banned LGBT propaganda toward minors and imposed an 18+ label on LGBT-related content. Last year, the law was updated to include "LGBT propaganda" aimed at any age, according to the outlet.
"The fines for 'LGBT propaganda' are usually enormous, sometimes equivalent to several monthly salaries," Sergeyev said. "But this is still far from a prison sentence for extremism. From now on, the risks will be as high as they are for Jehovah's Witnesses, Navalny supporters, or members of the Vesna activist group."
He warned that the most permissive understanding of the term "extremist" might enable law enforcement to initiate legal proceedings against individuals who have publicly affiliated with LGBT lifestyles or symbols.
"Considering this 'international LGBT organization' does not exist, the authorities might start arresting anyone related to LGBT in any way. Say, if you went to a gay pride event or posted a rainbow flag some years ago, you will be a potential target," Sergeyev said, according to Moscow Times.
The anti-LGBT policies of the Russian government are a component of a larger ideology that pits "Western" ideals like LGBT rights against "traditional" Russian values.
Furthermore, despite Russian leaders' claims that they are "at war with the collective West" in Ukraine, any local groups that support "Western" ideologies also need to be opposed.
Muslim officials in the Muslim-majority town of Chechnya, which is strictly heterosexual, also praised the Supreme Court ruling and took aim at the United States and Western civilization.
"Russia has shown once again that neither the collective West nor the United States will deprive us of the most important thing of all: a religious and national identity," said Minister Akhmed Dudaev, according to the paper.
Sergeyev believes that the Supreme Court's decision is an effort by the conservative Russian government to re-elect President Putin as the country heads toward the next presidential election in March.
"I believe this [the declaration of LGBT as extremists] is the government flexing its muscles before the elections. They’ll certainly put a lot of media weight on this to make sure the conservative electorate is watching the struggle for ‘traditional values.’ And LGBT people will be erased from the public sphere," said Sergeyev.