Coexistence requires that stabilizing niche differences, which cause species to limit themselves more than others, outweigh relative fitness differences that cause competitive exclusion. Interactions with shared mutualists, which can differentially affect host fitness and change in magnitude with host frequency, can satisfy these conditions for coexistence, yet empirical tests of mutualist effects on relative fitness and stabilizing niche differences are largely lacking within the framework of coexistence theory. Here, we show that N-fixing rhizobial mutualists mediate coexistence in four naturally co-occurring, congeneric legume (Trifolium) species. Using experimental greenhouse communities, we quantified relative fitness and stabilizing niche differences for each species in the presence of rhizobia originating from conspecific or congeneric hosts. Rhizobia stabilized coexistence by increasing self-limitation of Trifolium species grown with conspecific rhizobia, thus allowing congeners to increase when rare. Greenhouse-measured invasion growth rates predicted natural, unmanipulated coexistence dynamics of Trifolium species over two years in our field sites. Our results demonstrate that interactions with shared mutualists can stabilize coexistence of closely related species.