The coastguard helicopter was called out early on Sunday morning after a crewman aboard a fishing vessel had to be evacuated.
The man took ill shortly after 6am 84 nautical miles east south east of Sumburgh.
The name of the vessel has not been released.
Shetland Coastguard said the helicopter was tasked to collect the patient from the vessel.
He was taken to the Clickimin landing site in Lerwick, where he was met by a coastguard rescue team and transferred to the Gilbert Bain Hospital for treatment.