The call was made at 7.20am on Tuesday and has been traced to the Scapa Flow area.
Shetland Coastguard is currently responding and is conducting coastal searches to identify the source.
A spokesman for the coastguard said: “At 7.20am this morning, a spoken word Mayday was heard over the system.
“We broadcast a message to all vessels in the area and received no information back.“Two RNLI lifeboats from Stromness are currently conducting coastal searches to identify the possible source of the possible Mayday call.
“A search and rescue helicopter will also join the search.
“We are covering the area to a high degree.”