The mammal was photographed by Scots paramedic Danny Kerr off the west coast, between Islay and Gigha, as he was flying to pick up a patient.
He later sent the image to the marine charity Sea Watch, which enlisted the help of two American academics.
All three independently concluded that it was a Sei whale.
Adult Sei whales can weigh between 20 and 30 tonnes and grow up to 19.5m in length.
The mammals can live up to 65 years and swim up to 55km per hour.
Mr Kerr spotted the whale while the aircraft, from the Clyde Heliport in Glasgow, was flying at about 500ft.
"There is always excitement on board when the team spots a school of dolphins or other marine animals," he said.
"This time I was lucky, there were no patients on board and I had a camera …
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