Another 45 pilgrims were wounded in the attack, which struck their bus as it was passing through the town of Muqdadiyah en route from the Iranian border to the Shiite holy city of Najaf.
Najaf, which lies south of Baghdad, is home to the shrine of the first Shia imam.
Earlier in the May, at least five more Iranian pilgrims have been killed and 20 others injured after a car bomb hit their bus in north of Baghdad.
The bomb exploded on the highway from Balad to Samarra, a provincial police source said.
The attack occurred when a booby-trapped car hit the bus near the city of Samarra, some 110 km north of Baghdad.
More than 400 people have been killed and hundreds more injured in bombings and others acts of violence in Iraq since the start of May.
This comes as the United Nations Assistance Mission for Iraq (UNAMI) said on May 2 that April was the deadliest month in Iraq since 2008 as terrorist attacks killed over 700 people and injured more than 1,600 others across the country.