Brassinosteroids are phytohormones involved in plant development and physiological processes. Brassinosteroids Insensitive 1 (BRI1) is required for BR perception and initiation of subsequent signal transduction in Arabidopsis. In this study, the orthologue of BRI1 in the model legume species Medicago truncatula, MtBRI1, was identified and characterised. Three allelic Tnt1 insertion mutants, mtbri1-1, mtbri1-2, and mtbri1-3, were obtained from the M. truncatula Tnt1 insertion population. mtbri1 mutants displayed characteristic bri1 mutant phenotypes: extreme dwarfness, dark green curled leaves, short primary roots, less lateral roots, and insensitive to exogenous brassinolide (BL). Moreover, mtbri1 mutants show decreased total nodule number and defects in nitrogen fixation. MtBRI1 is able to complement an Arabidopsis BRI1 mutant, bri1-5. Similar to the interaction of BRI1 and BAK1 in Arabidopsis, MtBRI1 interacts with MtSERK1 in vivo. Global gene expression profiling revealed that the expression of BR biosynthesis genes and SAUR genes are significantly altered in mtbri1 mutants. MapMan analysis indicated that genes involved in signaling, hormone, cell wall, and biotic stress responses are over-represented in differentially expressed genes. Taken together, the results indicate that MtBRI1 is the BR receptor in M. truncatula and that BR signaling may play a conserved role in balancing plant growth and defenses.