Arbuscular mycorrhizal fungi (AMF) are known to improve plant growth and nutrition and therefore are likely to affect the competitive relationships between crops and weeds. In this study, we evaluated whether AMF (Funneliformis mosseae, Rhizoglomus fasciculatum, Rhizoglomus intraradices) change plant competition between Phaseolus vulgaris and the weeds Solanum nigrum L., Digitaria sanguinalis L., and Ipomoea purpurea L. Mycorrhizal colonization, aggressivity index, photosynthetic rates, and yield parameters were measured. While the presence of AMF reduced the total biomass of D. sanguinalis and S. nigrum when grown in competition with P. vulgaris, it increased the total biomass of I. purpurea when grown with P. vulgaris. Significantly, elevated mycorrhizal growth responses (38-44%) improved the competitive ability of I. purpurea. In contrast, the competitive ability of S. nigrum was increased only when plants colonized by R. intraradices. The total protein content of P. vulgaris pods when in competition was negatively affected by AMF, thus leading to low nutritional quality. The results suggest that AMF have the potential to affect the outcome of weed-P. vulgaris competition. We demonstrate that not only colonization with AMF but also AMF species can affect the competitive relationships between crops and weeds, and thus, AMF represent key soil organisms to be taken into account in sustainable weed management strategies.