A five-year field survey of the butterflies (Rhopalocera) of King County, Washington was conducted from August 2017 to August 2022 to compare the current butterfly fauna with historical records from the previous 140 years, and to expand our knowledge of butterfly diversity and distribution in this highly urbanized county. King County (5,478 km²) is the most populous county in the state and has a wide variety of ecosystems, including three Level III ecoregions. For this survey, 73 sites were sampled for butterfly species occurrence, from sea level to 1,700 m (5,600 ft) elevation and with multiple sites in each ecoregion. The species recorded during this survey were compared to those recorded historically (pre-1996) from King County in Hinchliff (1996), as well as with the sum of currently known King County records. Notable changes in species diversity and the impacts of urbanization are discussed. All records and literature pertaining to the species not recorded during this survey but known to have occurred in King County were reviewed. A total of 63 species were recorded, including six species and one subspecies which were first records for King County. A total of 87.7% of the species recorded in Hinchliff (1996) and 85.1% of all recorded King County species were found during this survey. The history of the literature relating to the study of butterflies of King County is also discussed, as well as the notable species found during this survey.