The Chestnut-sided Warbler (Setophaga pensylvanica) typically breeds in early successional deciduous forests of the northeastern United States and southeastern Canada. In recent decades, however, the species has become increasingly regular as a vagrant in the western U.S. and Canada. Over several weeks in June and July 2023, near Gig Harbor, Pierce County, we observed the first documented breeding behavior of the Chestnut-sided Warbler in Washington State, including territorial singing by the male and nest-building by the female. The increase in early successional vegetation following logging or wildfires could favor the Chestnut-sided Warbler becoming a regular breeding species in the Pacific Northwest.