T&R Productions LLC, a corporation based in Washington, registered with the Department of Justice under the Foreign Agents Registration Act as an agent for ANO TV-Novosti, which controls the RT network's worldwide broadcasts.
FARA is aimed at forcing transparency on lobbyists and lawyers working on behalf of foreign interests in Washington, and specifically exempts US and foreign news organizations.
Applying the law to T&R Productions, which operates US studios for RT and hires its staff, was seen as a way around that stipulation.
After being given a Monday deadline by the Justice Department to register, RT had originally said it would challenge the demand in court.
"Between a criminal case and registration we have chosen the latter. We congratulate American freedom of speech and all those who still believe in it," RT head Margarita Simonyan said in a caustic tweet.
The US pressed the issue after the Moscow-based broadcaster became a focus of the investigations into alleged Russian interference in the 2016 US presidential election.
A January 6 report by the heads of the major US intelligence agencies singled out RT as part of the Kremlin's campaign "to undermine faith in the US government and fuel political protest."
The Justice Department said in a statement that FARA "does not inhibit freedom of expression (and) does not restrict the content of information disseminated."
"Americans have a right to know who is acting in the United States to influence the US government or public on behalf of foreign principals," Acting Assistant Attorney General Dana Boente said.
But the move could provoke retaliation.
Moscow said measures requiring media outlets such as US government-funded Voice of America and Radio Free Europe/Radio Liberty to register as foreign agents could be passed by the Russian parliament as early as Wednesday.
"We will try to resolve this issue fairly quickly,"Pyotr Tolstoy, the deputy speaker of parliament's lower house, told journalists.
Members of parliament meanwhile have given contradictory statements on whether the Russian law could apply to commercial TV network CNN.
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