Five people were taken ill at Kendal Calling, in Cumbria, on Friday morning.
The man, who was found in a critical condition, later died, while a woman, 29, remains in a critical but stable condition, Cumbria Police said. Three men are "serious but stable".
A man, 20, has been held on suspicion of possession with intent to supply, the force said.
A man has died and four people are seriously ill after suspected "substance abuse" at a music festival.