Senior Congress leader Mullappally Ramachandran said the 47-year-old had stood unopposed for election as the president of the party his family had led for generations.
"I hereby declare Shri Rahul Gandhi elected as the President of the Indian National Congress," he told reporters outside the Congress headquarters in New Delhi.
"This is a historic occasion."
The announcement caps years of speculation that the scion of the Nehru-Gandhi dynasty would take over from his 71-year-old mother Sonia who has led the party for nearly two decades.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Mr Rahul, whose father, grandmother and great-grandfather all served as prime minister, has been her deputy since 2013 and led the campaign for the last general election.
He will officially take over on Saturday at a ceremony in New Delhi.
Congress party workers let off firecrackers and cheered as they celebrated the announcement.
"I have waited for this day for so many years. My joy knows no bounds," said Prem Chowdhury, a long-time party member and former municipal councillor.
Mr Rahul was strongly criticised for a lacklustre campaign for the 2014 general election, in which Congress recorded its worst-ever showing and lost power to Prime Minister Narendra Modi's Bharatiya Janata Party.
Since the national election defeat, the party has lost polls in many states to the BJP, exposing him to further criticism.
But few inside the party have been willing publicly to criticise the family that has been at its helm for generations.
