Ten of the 12 were officials at prisons where Magnitsky was detained, law enforcement and judiciary officials, and participants in the massive tax fraud that Magnitsky exposed and was killed over.
One participated in the 2009 Vienna murder of exiled Moscow critic Umar Israilov, and another took part in the 2004 murder of journalist Paul Klebnikov, who had probed Russian corruption, the US Treasury said.
The sanctions block them from traveling to the United States, freeze any US assets they have, and forbid Americans from doing business with them.
The were set under the 2012 Sergei Magnitsky Rule of Law Accountability Act, passed by Congress to pressure Russia over egregious human rights violations.
The act says Washington should impose sanctions on anyone seen responsible for the detention, abuse of death of Magnitsky, or who financially benefited from it.
But it also can be extended to anyone taking part in extrajudicial killings which the Treasury said the murders of Klebnikov and Israilov were or who engage in torture and other human rights violations against people seeking to expose illegal government activity.
Magnitsky spent 11 months in Russian jails without trial after he exposed the massive theft of state assets by Russian officials. He died in prison at age 37 after having been denied medical care for various ailments.
The US imposed sanctions on 18 Russian officials under the law in April 2013, to which Moscow responded by placing its own sanctions on 18 US officials.