The pilgrims were headed for a shrine in the city of Samarra Thursday, the same shrine whose destruction in 2006 by insurgents sparked a war between Sunnis and Shiites that left tens of thousands dead, The New York Times reported.
The pilgrims had begun their journey in Baghdad, and were passing through the city of Dujail, about 40 miles from Samarra, when the car bomb exploded.
The procession, thought to contain about 80 people, had stopped at a tent for water and a meal when the car exploded.
Officials said no one appeared to be in the car.
In January, 52 people were killed and at least 150 wounded when suicide bombers targeted Shiite pilgrims on a march to another religious site, in the city of Karbala.