Iran has not officially released any details about the specific charges against Mousavian but the semiofficial Fars news agency - which is deemed close to the country's elite Revolutionary Guards - said Mousavian could face espionage charges.
"The probable charge of espionage activities may be raised against him," the agency quoted an unnamed official as saying.
"Mousavian was arrested because of connections and exchange of information with foreign elements."
Fars said Mousavian was summoned to the prosecutor's office on Monday, where he was arrested and taken to the notorious Evin prison in northern Tehran.
"Mousavian's case is under initial investigation and interrogation, and his release is unlikely," Fars said.
Mousavian was a member of the Iranian nuclear negotiating team until 2005 and also served as Iran's ambassador to Germany in the late 1990s and early 2000s.