Heavy rains and thunderstorms accompanied by lightning strikes lashed the state over the past 24 hours.
Officials said most of the victims were farmers working in the fields when the lightning struck, Xinhua reported.
The meteorological department has forecast heavy rains and lightnings in different parts of the state in the next 24 hours.
Bihar Chief Minister Nitish Kumar has announced monetary relief for the victims' families.
"Chief Minister has announced an ex-gratia payment of INR 400,000 (5,070 US dollars) to the next of kin of the deceased," a local government spokesman said.
Kumar has also urged local residents to remain vigilant in bad weather and follow the advice issued by the disaster management department. "Stay at home in bad weather, stay safe," Kumar said.
Last week 17 people were killed in lightning strikes across the state.