Japan has cracked one of the world's great mysteries — how do you know if your cat is down?

Scientists in the land of Hello Kitty have come up with an app that can tell if felines are feeling poorly.

The "Cat Pain Detector" developed by Nihon University's College of Bioresource Sciences is able to detect if moggies are sick or off-form with "more than 90 percent" accuracy, its creators told AFP.

Vets are now using it in the cat-mad country, they claim, vaunting its AI technology trained on hundreds of thousands of cat photos.

The next frontier is knowing exactly what a cat actually wants so it can at last order a dry martini with an anchovy chaser.