The phytohormone auxin is a critical signal molecule, regulating fundamental processes in plant growth and development, such as shaping the root and shoot architecture, organ patterning, and nodulation. Auxin regulates plant gene expression mainly through auxin response factors (ARFs), which bind to auxin response elements in the promoter, upstream of auxin-activated genes. Here we examine and assess the function and expression pattern of a gene described as an auxin response factor, containing a DNA-binding pseudobarrel and B3 DNA-binding domains, from Medicago truncatula (MtARF-B3). For the model legume species M. truncatula, stable transgenic plants with MtARF-B3 overexpression, downregulation, and transcriptional reporters were constructed. Phenotypic and morphological evaluation of the obtained transgenic plants confirmed the important role of MtARF-B3 in general plant growth and development, modeling of root architecture, and development of seeds. Detailed histochemical and transcriptional analysis revealed expression of the gene in various stages of somatic embryogenesis, during formation of plant organs and tissues, and symbiotic nodulation. The fact that MtARF-B3 was strongly expressed in stamens and pollen grains in M. truncatula suggests that this gene could play a role in the fertility of this model legume.