Most of the victims were farmers in open fields and other workers caught in exposed areas, authorities said.
Local media reported the boy died with two others when lightning struck the group outside his house.
The seasonal storms also left several other people injured, with officials adding some were in a critical condition in hospital.
The intense lightning came amid storms and heavy rain which swept across several parts of the country, including the capital Dhaka.
A rise in deaths from lightning strikes in Bangladesh in recent years has been linked to deforestation.
Experts said deforestation has led to the disappearance of many tall trees which previously helped draw lightning away from people.
Lightning kills hundreds of people every year in Bangladesh.
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