Marc Mackay was last seen in the harbour area of Portrush around 1 am on Sunday evening.
The 24-year-old – who is also known as Adam – is described as being approximately 5ft 11ins, of slim build with short, spiked dirty fair hair.
He is believed to have been wearing a short-sleeved blue denim shirt and light blue denim jeans on the night he went missing.
Marc’s family and friends back in his home town of Dungannon, Co Tyrone are becoming increasingly concerned for his safety as every hour ticks away.
The PSNI would like to hear from anyone who saw or spoke with Marc in the early hours of Sunday morning or during Saturday night to contact them immediately on the non-emergency 101 number.