On the Hong Kong stock market, one of the exchanges where Rusal's shares are listed, it closed 50 percent down at HK$2.34.
The fall wiped more than 3.5 billion euros ($4.3 billion) off the market capitalisation of the company which is headed by billionaire Oleg Deripaska and accounts for some seven percent of the world's aluminium production.
In Moscow, the other exchange where its shares trade, they were down 23 percent in mid-session business.
The meltdown came after US President Donald Trump on Friday announced sanctions against Russian oligarchs close to Vladimir Putin following the diplomatic crisis sparked by the poisoning of former spy Sergei Skripal.
Washington has accused Deripaska of operating for the Russian government. Other magnates hit by sanctions include the director of state-owned energy giant Gazprom, Alexei Miller.
The Kremlin on Monday slammed the latest round of sanctions.
"This is an outrageous business from the point of view of illegality, from the point of view of flouting all the norms, and of course careful analysis is needed here," Putin's spokesman Dmitry Peskov told journalists.
Prime Minister Dmitry Medvedev said Moscow was looking at how to support the companies and sectors that have been hit.
In a statement, Rusal said the sanctions "may result in technical defaults in relation to certain credit obligations of the group".
The latest wave of sanctions also saw Russian stock market indices plummet around 10 percent.
The RTS index, which trades in dollars, fell 9.75 percent, and the MOEX index, which trades in roubles, fell 6.9 percent.
Russia's currency also took a hit, with the dollar and euro rising to 59.81 and 73.42 against the rouble respectively.
- Aluminium price surges -
Meanwhile aluminium prices surged on the London metals stock market (LME).
As of 1200 GMT, the price of aluminium had surged 3.97 percent to $2,123, its biggest jump in three years.
"Base metals are making a subdued start to the new week of trading -– with the exception of aluminium," Commerzbank Commodity Research said in a note.
"This is due to sanctions imposed by the US on Russian oligarchs and their companies on Friday," it added.
Matt France, global commodities broker at Marex Spectron, agreed: "No doubt about the outperformer in both price and volume as aluminium surges post the Friday Rusal announcement."
In all, Trump's administration targeted seven oligarchs, 12 companies they own or control, 17 senior Russian officials and a state-owned arms export company.
The sanctions follow weeks of tit-for-tat diplomatic measures between Russia and Western states following the poisoning of a Russia former double agent in Britain last month.
The health of Sergei Skripal, the former Russian spy who was found slumped on a bench with his daughter in the English city of Salisbury, is now improving.
The US said however that Friday's action was taken under a law passed to punish Russia for its alleged bid to interfere in the 2016 US presidential vote, as well as its intervention in conflicts in Syria and Ukraine.
- Countries: None
- Iran Warns of Dire Regional Consequences if Nuclear Deal is Canceled
- Russian spy attack: why Britain is limited in how it responds to Russia
- Sergei Skripal and the long history of assassination attempts abroad
- Cancer is costing BRICS economies billions each year
- Russia Issues Protected Mobile Phones to Top Officers