He was in a boat with another man when the boat capsized.
The other man managed to swim to safety to a nearby small island.
However one man is still missing.
The alarm was raised at around 2:20 this afternoon.
Valentia Radio station is coordinating the search involving the Shannon based helicopter, Waterville Coastguard, Derrynane Search and Rescue, and Castletownbere Coastguard and Lifeboat.
The search has been stood down for the night and will begin again in the morning.