The western spruce budworm (Choristoneura occidentalis occidentalis Freeman) is recognized as the most ecologically and economically damaging defoliator in western North America. Synchronous western spruce budworm outbreaks can occur over much of a host species’ range, causing widespread limb and tree mortality, regeneration delays, and reduction in tree growth rates. Observational outbreak records in northern Washington State extend back only to the mid-20th century, limiting our understanding of this species’ long-term population dynamics. In this study, we used dendrochronological methods to reconstruct multicentury outbreak records at four sites in the Okanogan Highlands of northeastern Washington State. We assessed long-term changes in outbreak patterns and tested moisture availability as a potential driving factor of western spruce budworm population dynamics. Outbreak synchrony was found to increase after the late 19th century, especially for high-intensity outbreaks, possibly due to anthropogenic factors. Moisture availability records show that outbreaks tend to occur at the end of droughts. As the variability of climate conditions is projected to increase, trending towards warm and dry summer conditions, the intensity and frequency of high-intensity western spruce budworm outbreaks may increase as well.