The whale, whose more normal habitat should be deep oceanic waters, was spotted at the weekend near Brixham’s Sharkham Point.
Conditions were quiet and the creature, judged to be 9m or 10m long, was swimming in an easterly direction up the English Channel.
It was intercepted by a pleasure boat run by Paignton Pleasure Cruises, which slowed and let the creature pass ahead, seemingly unperturbed as passengers took in the sight.
In a BBC report, a photograph shows just how close to the coast the whale had come.
It is possible that it moved inshore to get away from the English Channel’s busy and, to a whale, noisy shipping lanes.
The sighting may have been rare but, remarkably, the boat’s skipper, Ashley Lane, spotted a sperm whale in nearly the same place at nearly the same time last year.
There is conjecture that the two sightings may have been of the same creature, which would suggest that the whale's arrival off the South Coast this year was deliberate.
However, another possibility is that it is a different creature which has swum up the Channel by accident, its sonar-based navigation senses having become confused.
This, experts believe, can occur through exposure to noise from shipping or from underwater sonar or seismic activity.
Specialist groups will now be wanting to keep tabs on the whale’s movements if at all possible.
Any sighting where it is suspected that the whale could be injured, entangled, close to a stranding or in some other form of distress should be reported to British Divers Marine Life Rescue.
General information about the whale's movements and behaviour would be welcomed both by the Whale and Dolphin Conservation Society and by the Sea Watch Foundation.