The attack happened on Tyrella beach, outside Downpatrick in Co Down.
Detective Chief Inspector Darren McCartney of the PSNI Serious Crime Branch said: “The 22 year old victim sustained a serious head injury in a ‘one-punch’ type incident at around 7.30pm and is understood to be in a critical condition in hospital this morning.
“The arrested man is in custody at present assisting us with our enquiries.
“These ‘one punch’ incidents have a hugely destructive impact on both victim and perpetrator and the consequences can be devastating, completely altering the lives of all concerned.
“I am asking for any witnesses to this incident to get in touch.
“I am particularly keen to hear from anyone who was on the beach or in the carpark yesterday evening and who can assist police, they are asked to contact detectives on 101, quoting reference 1913 27/06/19.”