The relationship between the weed community and soybean (Glycine max (L.) Merr.) seed yield and quality was assessed in two soybean experiments in Illinois, USA. One field was sown with different proportions of target weeds (Ambrosia trifida L., Amaranthus rudis J. Sauer, Setaria faberi F. Herrm), and the other was naturally infested with these and other weeds. The composition of the weed communities in both fields were compared to final yield and quality (% protein, oil, and water) of the crop using NMDS ordination. Biomass and canopy cover, and seed quality (% protein, relative water content, seed weight) of the crop, were related to the multivariate structure of the weed community in both experiments. Lower quality soybeans were harvested from plots dominated by the target weeds and a suite of subordinate volunteers. Analysis restricted to the volunteer weed community was also significantly related to seed protein and seed weight. Similar results from the two experiments lend generality to the findings and indicate that soybean producers need to manage the composition of the weed community.