Atmospheric aerosol particles are key players in the Earth’s climate system, but there is still considerable uncertainty about where and how these particles are initially formed. We present the first study of new particle formation (NPF) at a pristine site in a subboreal forest region of North America. Our data suggest that, in this environment, there is frequent NPF from biogenic organic precursor compounds, which was likely the predominant source of particles in the preindustrial environment.