Dumri assembly seat in Jharkhand began at 7 a.m. on Tuesday, amid tight security arrangements, an official said.
Voting is underway in 373 polling stations, of which around 200 have been identified as Maoist-affected booths, he said.
Polling is scheduled to continue till 5 p.m.
Over 2.98 lakh voters, including 1.44 lakh women, are eligible to exercise their franchise in the by-election and will decide the electoral fate of six candidates, including three independents.
Heavy security arrangements have been made with the deployment of Jharkhand Armed Police and Central Reserve Police Force personnel along with home guards, the official said.
The bypoll was necessitated by the death of Jharkhand Mukti Morcha MLA and former education minister Jagarnath Mahto.
He had been representing the seat since 2004.
The JMM has fielded Mahto's wife Bebi Devi as the nominee of the opposition bloc INDIA, while the AJSU party has nominated Yashoda Devi as the NDA candidate. The AIMIM has also fielded Md Abdul Mobin Rizvi.
Votes will be counted on September 8.