The suspected plan to target a gathering of the National Council of Resistance of Iran (NCRI) in a Paris suburb came to light a few days after the June 30 event.
Six people were arrested in Belgium, France and Germany, two of whom were later released.
Belgian police believe the man arrested at a hotel in Paris, identified only as Merhad A. aged 54, is an accomplice of a husband and wife team caught in Brussels in possession of 500 grams of the powerful explosive TATP and a detonator.
The couple were identified as Amir S., 38, and Nasimeh N., 33.
All three are Belgian nationals of Iranian origin.
Separately, German police swooped on an Iranian diplomat based in Vienna who has been named as Assadollah Assadi.
German prosecutors believe Assadi, believed to be an intelligence agent, ordered the couple to attack the rally and had handed them the explosives at a June meeting in Luxembourg.
Tehran has dismissed the alleged bomb plot as a “sinister false flag ploy” designed to discredit Iran at a time when it faces major diplomatic tensions with the United States.
The rally in the Paris suburb of Villepinte was attended by several allies of US President Donald Trump, including former New York mayor Rudy Giuliani and former House speaker Newt Gingrich, both of whom urged regime change in Iran.
Belgian authorities requested the extradition of both Assadi and Merhad A.
Merhad A. was questioned by French police for three days before being taken into custody. He has agreed to he handed over to Belgium.
He was arrested by police acting on an anonymous tip-off. In his phone he had an Austrian SIM card with just one Austrian number on it.