We report the first occurrence in the Pacific Northwest, USA, of the harpacticoid copepod genus Harpacticella (Family Harpacticidae), which has previously been recorded only from Asia and from an atoll in the Indian Ocean. Harpacticella paradoxa (Brehm, 1924) is native to streams, lakes and estuaries in Japan and China. H. paradoxa was found in plankton samples taken in 2000 and 2004 from 2 geographically separated estuaries (Samish and Klamath River Estuaries, USA) at salinities ranging from 0 to 15 psu. H. paradoxa was also found associated with plants sampled in 2006 from a freshwater reservoir in the Columbia River, Washington State, USA. Because the genus Harpacticella has previously been found only in Asia and the Indian Ocean; is probably easily transported on ship hulls or in ballast; and appears to have a disjunct regional distribution, we regard it as a probable introduced species to North America.