The Justice Department announced on Friday that all 12 of the defendants were members of the GRU, a Russian Federation intelligence agency within the Russian military.
A grand jury convened by Mueller accusing the Russians of engaging in a “sustained effort” to hack Democrats’ emails and computer networks.
Deputy Attorney General Rod Rosenstein said no American citizen was indicted.
“There is no allegation in this indictment that any American citizen committed a crime,” he said at a news conference. “There is no allegation that the conspiracy altered the vote count or changed any election result.”
The indictment comes as President Donald Trump is scheduled to meet Russian President Vladimir Putin in Helsinki on Monday.
Mueller, a former FBI director, has been running a high-profile investigation into allegations that Trump won the 2016 US presidential election against Democratic rival Hillary Clinton only because Moscow had rigged the election in his favor.
US intelligence agencies claim Russia-linked hackers provided WikiLeaks with damaging information — in the form of thousands of hacked emails — about Clinton to skew the 2016 presidential election in favor of Trump.
Trump has repeatedly denied allegations that his campaign colluded with Russians and has condemned the investigations. Russian President Putin has also denied the allegations.