The incident happened amid stormy weather at around 2pm on Tuesday, with strong westerly winds of between force six and seven.
Crews from Kilkeel RNLI said the surfer had been in the Lough for around an hour when they found him and his board.
He was rescued and the board - which couldn't be retrieved due to the rough conditions - was towed into Greenore.
He was described as being in "reasonably good health" despite how long he was in the water.
Roy Teggarty from the RNLI praised the actions of the lifeboat crews.
"This was a day with difficult conditions because of the strong winds," he said. "It was mainly difficult to keep the lifeboat steady when retrieving the casualty so this rescue involved expert boat handling by all involved."