Japanese sweet potato is different from American sweet potato or yam. Sometimes it’s called Asian sweet potatoes. The outer skin is dark pink, and inner flesh is yellow or cream color, and the water content is much lower and sweeter than orangey yam. They are still not commonly available in regular American grocery stores, but are available at health food stores, Japanese grocery stores, Korean and Chinese grocery stores.
The sweet potatoes are native to South America, but came to Japan through China in the 17th century. Because it yields a lot in poor soil, the cultivation was encouraged by the government until after the WWII.
Sweet potatoes can be roasted, steamed, sliced and half dried, sliced and fried as tempura (shojin-age), and used for many Japanese confectioneries. Shochu, Japanese distilled alcoholic beverage, can be made from sweet potatoes.