Oxalis illinoensis, the Illinois woodsorrel, is a species of flowering plant in the woodsorrel family. It is native to the United States, where it is endemic to the Interior Low Plateaus of Illinois, Indiana, Kentucky, and Tennessee. The limits of the range of this species are unclear due to its similarity to Oxalis grandis, with which there has been confusion. Oxalis illinoensis is a perennial that produces yellow flowers with red-lined centers. Its primary habitat is calcareous forests and bluffs.