Mousavi told the Islamic Association of the Iranian Medical Society the party's name is the Green Path of Hope Association, Press TV reported.
"The color green is the symbol of this movement; its slogan is demanding the impeccable implementation of the constitution, and innumerable self-motivated independent societies form the body of this movement," Mousavi said.
Mousavi is a former prime minister who was close to Ayatollah Ruhollah Khomeini, the founder and first Supreme Leader of the Islamic Republic.
President Mahmoud Ahmadinejad was re-elected June 12 in an election many observers say was blatantly rigged. The official results gave the president a 2-1 margin around the country, even in areas where Mousavi had a lot of support.