Matthew Bryce, 22, from Glasgow, had not been seen since he set off to go surfing off the Argyll coast at 09:00 on Sunday morning.
Belfast Coastguard said he had been in the water since 11:30 on Sunday and was found at 19:30 on Monday.
A spokeswoman said Mr Bryce, who was wearing a wetsuit, was hypothermic and had been taken to hospital in Belfast.
Dawn Petrie, from the coastguard, said: "He was extremely lucky.
"Hope was fading of finding the surfer safe and well after such a long period in the water and with nightfall approaching we were gravely concerned but at 7.30pm tonight, the crew on the coastguard rescue helicopter were delighted when they located the man still with his surf board and 13 miles off the coast.
"He was kitted out with all the right clothing including a thick neoprene suit and this must have helped him to survive for so long at sea. He is hypothermic but conscious and has been flown to hospital in Belfast.
She told BBC Radio: "He did the right thing by staying with his surfboard and that certainly aided his survival."
Belfast Coastguard co-ordinated the search for Mr Bryce who left to go surfing from Machrihanish beach near Campbeltown.
A large area of sea and shore line was combed after the alarm was raised at lunchtime on Monday.
It involved RNLI lifeboats from Campbeltown, Islay and Red Bay and Coastguard rescue teams from Campbeltown, Southend, Gigha, Tarbert and Port Ellen as well as the Coastguard rescue helicopter based at Prestwick.
Mr Bryce had last been seen in the St Catherines area of Argyll.
An "extremely lucky" surfer has been found by a coastguard helicopter after 32 hours in the sea.