New York-based Human Rights Watch said the defendants had limited access to lawyers and did not get a fair trial.
The nine were among 24 defendants charged with the plotting against the state in proceedings that lasted two days this month in Ahvaz, capital of Khuzestan province, said a defense lawyer, Saleh Nikbakhat.
The bombings last June, October and January in Ahvaz killed 23 people and wounded dozens.
Arab Iranians make up less than 3 percent of Iran's population and live mostly in Khuzestan, in southwest Iran.
A Web site claiming to represent Arab secessionists in the Ahvaz region said they carried out the Jan. 24 attack, in which bombs exploded in a bank and outside a government building, killing six people and wounding 46 others. President Mahmoud Ahmadinejad was due to visit Ahvaz that day, but canceled the trip a few hours beforehand.
The Web site said the attack was "revenge for the blood of our martyrs and (to signal) our rejection of the terrorist Ahmadinejad's defiling the land of Ahvaz in his visit."
On June 8, the court found nine defendants to be enemies of God, and sentenced them to death, Nikbakhat said.
The other 15 defendants received sentences ranging from seven to 30 years in prison, Nikbakhat said.