The bomb targeted Shiite pilgrims returning from Karbala, an Interior Ministry official said.
Musayyib is located 50 miles south of Baghdad.
The attack, which injured 37, comes amid rising sectarian tensions.
This week, tens of thousands of Sunnis participated in nationwide protests against what they say is second-class treatment by Iraq’s Shiite-led government.
Sunnis largely boycotted Iraq’s 2005 elections, leading to the emergence of a Shiite-led government. The move left the once-ruling minority disaffected, which contributed to years of bloody insurgency and sectarian warfare.
Several apparent sectarian attacks were reported Monday, leaving 16 dead.
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