The cat-like cloud was photographed above Mount Kirishima on the border between Miyazaki and Kagoshima Prefectures on the southern Japanese island of Kyushu.
Kirishima has been erupting since 1st March, at one point belching clouds of ash 4,500 meters (14,763 feet) high.
The volcano has also been emitting rarely heard rumblings, local media report.
But the most talked-about thing to come out of the volcano in Japanese media has been the cat-shaped cloud with its well-defined triangular ears and symmetrical paws.
The pictures spread like wildfire, with internet users having great fun wondering what the giant sky-cat could mean.
One wrote: “This is awful! Giant cats are laughing at me.”
Another one asked: “Are the stars finally ruling the Earth?”
Cats are revered in Japan as bringers of good luck and figurines of a beckoning golden feline are widely seen in the country.
Japanese geologists say the country can expect a far more serious event within the next few years though.
It is reported locally that a “major earthquake in the (nearby) South China Sea” is likely within two years of the 2020 Summer Olympics, to be held in Japanese capital Tokyo.