The Interior Ministry said the bomber, disguised as a police officer, struck near a Shiite mosque where many pilgrims were going in observance of a holy day, CNN reported.
Since U.S. troops pulled out of Iraq last month, scores of pilgrims have been killed and wounded in attacks blamed on Sunnis.
Saturday's bombing, which targeted pilgrims heading to a Shiite mosque between Zubair and Basra, came on the last of the 40 days of Arbaeen, when hundreds of thousands of Shiite pilgrims visit the city of Karbala or other holy sites.
Arbaeen marks the seventh-century death near Karbala of Imam Hussein, the grandson of the Prophet Mohammed. Hussein's death in a battle for control of the Muslim nation resulted in a split between Sunni and Shiite Muslims.
The Los Angeles Times said thousands of pilgrims who cannot make the longer journey to Karbala, where the imam is buried, instead go to the Khotwa mosque outside Zubair.