The target list now includes 44 names of those whose assets under US jurisdiction are frozen
The US Treasury on Monday added Lugovoi along with Investigative Committee chief Alexander Bastrykin and Dmitri Kovtun -- also a suspect in the Litvinenko murder -- to the Magnitsky Act sanctions list.
"I am perplexed," Lugovoi was quoted as saying by RIA Novosti. He added he didn't know why he was on the blacklist only that he heard "persons who violated people's rights in some way" ended up on it.
"I think that (US President Barack) Obama is now rushing before handing over his prerogatives to harm and spite Russia in any way he can, and this has led to absurd things."
US State Department spokesman John Kirby said Monday the new additions to the sanctions list have "roles in the repressive machinery of Russia's law enforcement systems, as well as individuals involved in notorious human rights violations."
Litvinenko, an ex-spy turned Kremlin critic, died of radiation poisoning in 2006 aged 43, three weeks after drinking tea laced with radioactive polonium at an upmarket London hotel.
An inquiry last year found that Lugovoi and Kovtun, identified as prime suspects by British police, were likely to have carried out Litvinenko's poisoning on the instructions of the Russian security services.
Lugovoi has dismissed the allegations as "nonsense."
Obama's outgoing administration has accused the Kremlin of orchestrating cyber attacks aimed at influencing the results of November's White House race.
Moscow has vehemently rejected the accusations, over which Washington has already expelled 35 Russian diplomats allegedly involved in espionage and due to what Obama said was "harassment" of US diplomats in Russia.
The Magnitsky Act was originally passed to allow US officials to impose sanctions on Russians involved in the 2009 prison death of Russian tax fraud whistleblower Sergei Magnitsky.
But more individuals have been blacklisted over the years.
Opposition politician Boris Nemtsov, who was gunned down near the Kremlin in 2015, had petitioned US congressmen to expand the list with individuals seen as violators of human rights, including Bastrykin, who reacted at the time by saying that it "would be a great honour".
The target list now includes 44 names of those whose assets under US jurisdiction are frozen, and who are barred from doing business with Americans or receiving US visas.