Allium fistulosum, the Welsh onion, also commonly called bunching onion, long green onion, Japanese bunching onion, and spring onion, is a species of perennial plant, often considered to be a kind of scallion.

The species is very similar in taste and odor to the related common onion, Allium cepa, and hybrids between the two (tree onions) exist. A. fistulosum, however, does not develop bulbs, and possesses hollow leaves (fistulosum means "hollow") and scapes. Larger varieties of A. fistulosum, such as the Japanese negi, resemble the leek, whilst smaller varieties resemble chives. A. fistulosum can multiply by forming perennial evergreen clumps. It is also grown in a bunch as an ornamental plant.