A police spokesman said the incident happened at The Southern Sunset Festival, which took place at The Oval, Hastings, on Saturday (August 7).
A man in his 30s required stitches after allegedly being struck over the head with an item following an altercation at around 6.20pm, police said.
On Monday (August 9), a spokesman for Sussex Police said a 28-year-old man from Surrey was arrested on suspicion of ‘grievous bodily harm with intent and supplying a psychoactive substance’.
He has been released on bail subject to conditions while enquiries are ongoing, police said.
The spokesman added: “Anyone who witnessed the incident or has any information which could help with enquiries is asked to call 101 quoting serial 1162 of 07/08.”