The cultivation of cannabis on Western United States public lands has emerged as a critical environmental concern with profound ecological ramifications. These clandestine, illicit sites are established in geographically isolated areas that support sensitive habitats. The ecological disturbances and environmental pressures from these sites have been briefly described in limited media, public outlets, and scientific publications. However, descriptive and quantitative data on the amount of illegal pesticide use, water diversions, deforestation, and dead wildlife associated with these operations have not been reported to date. In this study, 541 illegal grow sites were documented on public lands in California, Oregon, and Nevada. A mean of 4.56 illegally used pesticides, 404 kg of applied soluble fertilizer, 12,068L of diverted water, 2064 m2 of cleared native vegetation, and 0.38 dead animals, occurred per site. Dead wildlife included cases of state and federally listed wildlife poisoning and evidence of food-web contamination. These data establish the foundation for understanding the potential ecological consequences of the thousands of remaining sites on western public landscapes and provides land management agencies with knowledge of potential effects on wildlife, biodiversity, water, soil, and ecosystem health to develop effective solutions to this ongoing problem.