Berberis thunbergii is a thorny shrub, 1 to 1.5 (rarely 2) meters tall with simple, entire, spatulate leaves. The leaves are alternately arranged, but this may be difficult to see because 2 - 6 (+) leaves are often clustered close together on spur branches immedately above each thorn. The location of the thorns and winter buds clearly reveal the alternate arrangement on this species. The thorns are usually 4 - 10 mm long, sharp and stiff. Flowers have 6 pale yellow to whitish petals first opening in May and the fruit is an elongate red berry with one or a few seeds, often remaining on the branches until November.

Japanese barberry is a native of Asia. It was introduced into North America as an ornamental and is now established outside of cultivation throughout the northeastern U.S.A. In some areas it is clearly an invasive species and appears to be spreading in Wisconsin forests, abandoned fields, fencerows, etc.