Jaleshwar Mahato, former Jharkhand minister and state president of the Janata Dal (United), joined the Congress on Saturday.
Congress president Rahul Gandhi welcomed Mr Mahato to the party, it said.
Mr Mahato was a JDU Member of Legislative Assembly or MLA from the Baghmara Vidhan Sabha constituency from 2000-09. His joining the Congress is seen by political analysts as a blow to the Nitish Kumar-led JDU in Jharkhand.
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.