The five mortar rounds slammed into a busy market, a residential building and a parking lot around 7 pm (1700 GMT, 11 a.m. EST) as people returned home from work and shopped in Musayyib, police and hospital officials said.
Musayyib is in an area that holds a volatile mix of Sunnis and Shiites
Police said it appeared the rounds came from the nearby Sunni-dominated town of Jurf al-Sakr, though it wasn't immediately clear who fired them. The officials gave the casualty toll and details on condition of anonymity because they weren't authorized to release information to journalists.
Musayyib, about 60 kilometres south of Baghdad, is in an area that holds a volatile mix of Sunnis and Shiites and was a flashpoint for some of the worst sectarian violence in past years.
On Tuesday, a parked car bomb in the town killed five civilians and wounded 13, authorities said. In 2009, a female suicide bomber targeted Shiite pilgrims there, killing at least 40 people. Nobody immediately claimed responsibility for today's attack, but Sunni insurgents frequently target Shiite areas and security forces. Violence has increased in Iraq amid Sunni anger over perceived mistreatment and random arrests by the Shiite-led government.
Last year, Iraq saw the highest death toll 8,868 since the worst of the country's sectarian bloodletting began to subside in 2007, according to United Nations figures.