Costea, M. (University of Guelph, Department of Plant Agriculture, Guelph, ON, N1G 2W1, Canada; coste_amihai@hotmail.com) and F. J. Tardif (University of Guelph, Department of Plant Agriculture, Guelph, ON, N1G 2W1, Canada; ftardif@uoguelph.ca). Taxonomy of the Polygonum douglasii (Polygonaceae) complex with a new species from Oregon. Brittonia 57: 1–27. 2005.—The taxonomy of the Polygonum douglasii complex is revised. A summary of relevant taxonomic information is provided, along with a key to the species and subspecies, plus typification, synonymy, discussion, distribution, and ecology. Morphology and micromorphology of stems, leaves, perianth, pollen, and achenes are analyzed and illustrated. Based on these data, 12 species and two subspecies are recognized. A new species, P. gabrielae, is described and illustrated from Oregon.