Dozens more were injured across the two states, according to officials.
Lightning strikes are fairly common during India’s annual monsoon season, which lasts from June to September.
But Lakshmeshwar Raj, Bihar’s disaster management minister, said this was one of the highest death tolls the state had recorded in recent years.
Most of those killed in Bihar were farm workers, struck down while planting seeds outdoors, government official Upendra Pal said.
Mr Pal said at least 10 others were injured and were receiving hospital treatment.
Bihar is a “multi-disaster prone state” which is “vulnerable to many hazards”, according to the state's Disaster Management Authority.
Monsoon season brings rain which is essential for the harvest in Bihar and across South Asia.
But the state suffers in heavy rainfall each year, with flooding in Bihar being responsible for 30-40 per cent of all flood damage in India.
Heavy rain and thunderstorms are expected across Bihar and eastern Uttar Pradesh on Friday and over the weekend, according to India’s Meteorological Department.
Prime minister Narendra Modi responded to the deaths on Twitter, writing: “I express my condolences to the families of those who have lost their lives in this disaster.”
The prime minister also said that state governments were responding with urgency to the storms.
More than 2,300 were killed by lightning in India in 2018, according to the most recent figures available from the National Crime Records Bureau.
Additional reporting by Associated Press
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