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.