It is a variety of Brassica rapa, the plant species that yields the turnip, mizuna, napa cabbage, and rapini.
The name komatsuna means 'greens of Komatsu' in Japanese, a reference to the village of Komatsugawa [ja] in Edogawa, Tokyo, where it was heavily grown during the Edo period.
[1] It was named by Tokugawa Yoshimune, the eighth shogun, who visited Edogawa in 1719 for hunting and stopped at the local Katori Shrine for lunch.
It grows faster and has bigger but fewer leaves than ordinary komatsuna, and a strong, spicy flavor.
It is a very versatile vegetable and can be eaten raw, pickled, stir-fried, boiled, used fresh in salads, or added to soups.