A jury in Manhattan federal court Friday also found Richard Chichakli guilty of money laundering and other charges.
Bout is serving a 25-year sentence for a 2011 conviction on charges he conspired to sell millions of dollars of weapons to Colombian rebels. He said he was a legitimate businessman.
Prosecutors say Chichakli helped Bout manage a network of aircraft companies since the mid-1990s.
Bout was banned from doing business in the United States in 2004 because of his role in pouring arms into violent conflicts. Prosecutors allege Bout and Chichakli violated sanctions by forming a front company that contracted to buy two Boeing airplanes.