All higher plants show developmental plasticity in response to the availability of nitrogen (N) in the soil. In legumes, N starvation causes the formation of root nodules, where symbiotic rhizobacteria fix atmospheric N2 for the host in exchange for fixed carbon (C) from the shoot. Here, we tested whether plastic responses to internal N of legumes are altered by their symbionts. Glasshouse experiments compared root phenotypes of three legumes, Medicago truncatula, Medicago sativa and Trifolium subterraneum, inoculated with their compatible symbiont partners and grown under four nitrate levels. In addition, six strains of rhizobia, differing in their ability to fix N2 in M. truncatula, were compared to test if plastic responses to internal N were dependent on the rhizobia or N2‐fixing capability of the nodules. We found that the presence of rhizobia affected phenotypic plasticity of the legumes to internal N, particularly in root length and root mass ratio (RMR), in a plant species‐dependent way. While root length responses of M. truncatula to internal N were dependent on the ability of rhizobial symbionts to fix N2, RMR response to internal N was dependent only on initiation of nodules, irrespective of N2‐fixing ability of the rhizobia strains. Most plants respond to changes in nitrogen availability with plastic responses in root and shoot architecture and allocation. Legumes can additionally form nitrogen‐fixing nodules, but it is not known how nodulation impacts on other phenotypic responses to nitrogen in legumes. This study shows that the presence of nodules can alter root length and root mass ratio responses to nitrogen and that these responses were dependent or independent of nitrogen fixation from the nodules, respectively. This finding was specific for the model legume Medicago truncatula, with different responses in clover and alfalfa.