The Olympic National Park (ONP) consists of 3,735 km2 of wilderness on the Olympic Peninsula in northwestern Washington. The park covers most of the Olympic Mountains range with its numerous glacier-covered peaks reaching more than 2,000m above sea level and alpine parkland and meadows, as well as forested valleys, rivers and lakes, rain forest, and coastal habitats. We conducted a one-year field survey of the Lepidoptera fauna of the ONP using a combination of day collecting, blacklight traps, and collecting at lights in early morning. A total of 369 species were encountered, which combined with a voucher collection at the Park headquarters, yields a checklist of 413 species records for the Park. While most species are common and widespread in the West or Pacific Northwest, the list also contains a few subspecies of butterflies that are endemic to the Olympic Mountains as well as several species that represent isolated disjunct relicts of arctic/alpine species found in the Cascades, Rockies, and/or far north. Another set appear to constitute the northernmost populations of species known from coastal California and Oregon. While our study begins to outline a rich diversity of Lepidoptera, it is clear that future studies, particularly in more remote areas of the ONP, will be necessary for a more complete view of this fauna.