Cyclachaena xanthiifolia (syn. Iva xanthiifolia), known as giant sumpweed, or rag sumpweed is a North American plant species in the sunflower family, Asteraceae. It is the only species in the genus Cyclachaena. Giant sumpweed is believed to be native to the Great Plains but is now found across much of southern Canada and the contiguous United States, though rarely in the Southeast.

It is an annual herb roughly 45–185 centimetres (18–72 inches) tall with stout, erect, branched stems. The stems are erect or ascending, finely ridged or grooved, and grayish-green. They are usually unbranched but sometimes have straight, upright branches. They are hairless near the base and are sparsely to moderately covered with fine curved or curled hairs above the base.

In Britain and Ireland, it is known as marsh-elder. It has been introduced and semi-naturalized in central and southern Britain, partly via seeds bought for bird food.

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