The spokesman for Babil province police, Maj. Muthana Khalid, said the buses carrying Iranian and Iraqi pilgrims overturned in the southern city of Hillah, about 60 miles south of the capital.
Two Iraqis were also killed, and more than 50 Iraqis and Iranians were injured.
Dr. Saad Al-Nakaash of Hillah hospital confirmed the casualties.
Shiite pilgrims come from all over the world to visit shrines and mosques in Iraq that are revered by Shiites, but the vast majority of the religious tourists are Iranians.
Both Iran and Iraq are Shiite majority countries. But before the 2003 U.S.-led invasion, relations between the Iranian government and Saddam Hussein's minority Sunni dominated country were so strained that it was much more difficult for Iranians to visit what are considered some of the most holy sites for Shiites.
Iran and Iraq fought a brutal eight-year war in the 1980s that left hundreds of thousands of people dead on both sides.
After Saddam was deposed, Iranians flooded into the country to see such locations as the Imam Hussein and Imam Abbas mosques in Karbala and the Imam Ali mosque in Najaf, and their tourist dollars became an important source of income for Iraq.
The pilgrims have often been targeted by Sunni extremists who view Shiites as nonbelievers, but Friday's crash did not appear to be related to violence.