Mark Vartanyan was extradited last December from Norway where he worked as the chief technical officer for e-healthcare firm Dignio.
Known online as "Kolypto", he is alleged to have developed, improved, maintained, and distributed Citadel from August 2012 while living in Ukraine, and continuing to do so in 2014 after he moved to Norway.
US prosecutors said Citadel was for sale on invite-only Russian language crime forums from 2011.
The malware was designed to steal financial account credentials and personal information while disabling anti-virus programs.
It is believed to have infected over 11 million computers worldwide, causing losses of over US$500 million (A$652 million).
Banks and financial institutions targeted by Citadel included American Express, Bank of America, Citigroup, Credit Suisse, PayPal, HSBC, JPMorgan Chase, Royal Bank of Canada, and Wells Fargo.
A joint operation including Microsoft and law enforcement in several countries took down the Citadel botnet in 2013.
Vartanyan was first thought to be "Aquabox", the original developer of Citadel who released the malware in 2011. This turned out to be incorrect and "Aquabox" is still at large.
Russia has protested Vartanyan's extradition to the US, saying it goes against international law.
Another Russian man, "RainerFox" or Dimitry Belorossov, received a four-and-a-half-year prison sentence in the United States in 2015 for using Citadel to infect over 7000 computers worldwide.
Earlier this week, US authorities indicted three Russian men and one Canadian over the massive Yahoo data breach in 2014. Two of the men, Dimitry Dokuchaev and Igor Suschkin, work for Russia's Federal Security Service FSB.
Russian Alexsey Belan and former Kazakhstani national Karim Baratov, who now holds Canadian citizenship, were also indicted. Of the four, only Baratov has been arrested.
The US and Russia are squabbling over another cybercrime extradition case, involving Russian Yegenyiy Nikulin, alleged to be the mastermind behind the 2012 data breaches that affected professional networking site LinkedIn and cloud storage company Dropbox.
Nikulin, a multimillionaire entrepreneur, is currently being held by Czech authorities in Prague, with both the US and Russia requesting his extradition.