The alert was raised at 5.20pm on Wednesday after the diver failed to return to a boat in the strait.
Two helicopters, three lifeboats and rescue teams were involved in the search for the diver, who was located two miles east of Duncansby.
He was brought to shore by an RNLI lifeboat and taken to Wick Hospital on the mainland.
Shetland Coastguard coordinator Gary Harris said: "During search operations at sea, it's incredibly challenging to spot a head in the water especially at night.
"He is reported to be conscious and in a reasonable condition considering he has spent 11 hours in the water."
The rescue comes three days after a surfer was pulled from the water 30 hours after going missing off the Argyll coast in western Scotland.
Matthew Bryce, 22, was wearing a full wetsuit, which was credited with saving his life, and had managed to stay with his board to keep him out of the water.
SKY News.