Caltha palustris (Kingcup, Marsh Marigold) is a herbaceous perennial plant of the buttercup family, native to marshes, fens, ditches and wet woods.
The plant is growing to 80 cm tall. The leaves are rounded to kidney-shaped, 3-20 cm across, with a bluntly serrated margin and a thick, waxy texture. Stems are hollow.
The flowers are yellow, 2-5 cm (1-2 in) diameter, with 4-9 (mostly 5) petal-like sepals and many yellow stamens; they are born in early spring.
In North America Caltha palustris is sometimes known as cowslip. However, cowslip more often refers to Primula veris, the original plant to go by that name. Both are herbaceous plants with yellow flowers, but Primula veris is much smaller.

Photographed in Hadashville, Manitoba, Canada by my husband. I wasn’t comfortable going into the ditch with my broken ankle just yet.