Oenanthe sarmentosa is a species of flowering plant in the family Apiaceae known by the common name water parsley.

It is native to western North America from Alaska to California, where it grows in wet areas, such as streambanks.

The plant has been used in cultivation in wetlands, and the recent discovery of several colonies growing by a stream in Illinois demonstrates its capacity to become a noxious weed if it is introduced elsewhere.

The parsley-like leaf blade is divided into serrated, lobed leaflets.

The inflorescence is a compound umbel of many flowers with bright white to red-tinged petals.