Manssor Arbabsiar, 56, is facing a five-count indictment. He's accused of conspiring to hire hit men from a Mexican drug cartel to kill the ambassador at a restaurant in Washington.
U.S. officials have indicated that agents of the Iranian government may have been involved in the plot, a charge Iranian authorities call "baseless."
Arbabsiar was arrested Sept. 29 at a New York airport on charges that carry a potential life sentence. He was held without bail.
U.S. authorities say they secretly recorded conversations between Arbabsiar and an informant with the Drug Enforcement Administration. They said Arbabsiar offered $1.5 million for the death of the ambassador.
Authorities said Arbabsiar eventually made a $100,000 down payment wired from an overseas account for a plot that would kill the ambassador at his favorite restaurant.
No trial date has been set. Arbabsiar's next court appearance is scheduled for Dec. 21.
The Associated Press contributed to this report.