A tragedy struck near Winchester, Hampshire, between 8 and 9 p.m. on Saturday.
The man was brought to the hospital and died the next day.
A spokeswoman for Hampshire and Isle of Wight Police said: “Police were aware of a reported medical incident at Boomtown Fair between 8pm and 9pm on Saturday, August 10.
“A man in his 20s was transferred to Royal Hampshire County Hospital where he was subsequently pronounced dead in the early hours of Sunday, August 11.”
His next of kin has been notified, according to authorities.
In a statement, a Boomtown spokeswoman added: “With sadness, we confirm a man in his 20s died on the evening of Saturday, August 10, after attending Boomtown.”
“Our thoughts are with his family, friends, and everyone who knew him.
“We are supporting the police with their inquiries, and with respect for his family, no further comment will be made.”
The five-day festival ended on Sunday night.
Over 450 performers performed from Wednesday to Sunday, including Damian ‘Junior Gong’ Marley, son of the late famous Bob Marley.
It is one of the largest festivals in the UK, attracting tens of thousands of people.
The event is typically hosted at the Matterley Estate near Winchester.