The search is from near the Menai suspension bridge, one of two bridges which links the mainland to Anglesey Rescuers are searching the Menai Strait between Anglesey and the north Wales mainland after reports in the early hours of someone in the water.
Emergency services had the call at 0324 GMT and are searching eight miles between Menai Bridge and Caernarfon.
Holyhead coastguard said team members had heard shouting in the water.
Coastguard teams from Bangor and Penmon, Beaumaris RNLI lifeboat, North Wales Police and search and rescue dogs are all involved.
Barry Priddis, watch manager with Holyhead Coastguard, said the search was from the Swellies – the area of water between the Britannia and Menai Bridges – to Caernarfon.
“The tide has just turned and as the search was concentrated towards Caernarfon it is now in the other direction,” he said.
He said further inquiries were also being made about the possibility of a man who had gone missing before.
Gareth Parry, operations manager for Beaumaris Lifeboat, said the weather was not causing any problems for the search.
He said there was light rain but visibility was fine.
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