Basic helix-loop-helix (bHLH) transcription factors (TFs) are known to play significant roles in plant growth and development, secondary metabolite synthesis, and stress responses. However, the functions of soybean [Glycine max (L.) Merr.] bHLH TFs have been less widely explored. The main purpose of this study was to explore the function of a bHLH TF subfamily gene, GmbHLH3, by overexpression in wild type Arabidopsis thaliana (L.) Heynh. GmbHLH3 is a homolog of Arabidopsis bHLH3 (AtbHLH3) that participates in the regulation of jasmonic acid (JA) signaling. Several cis-regulatory elements responding to methyl-jasmonate (MeJA), including G-Box CGTCA and TGACG motifs, were found in the promoter of GmbHLH3. GmbHLH3 is located in the nucleus. It can form dimers with AtbHLH3, AtbHLH13, and GmbHLH3a, respectively, by yeast two-hybrid and bimolecular fluorescence complementation assay. GmbHLH3 induced root growth by suppressing expression of AtPDF1.2, AtVSPP1, AtLOX2, and AtTAT1 under MeJA treatment. GmbHLH3 inhibited JA-induced leaf senescence by limiting transcription levels of the senescence-related genes AtSAG29, AtAZF2, AtWRKY22, and AtMKP2, and the chlorophyll degradation genes AtSGR, AtNYC1, AtCLH1, AtPPH, and AtPAO. Under long day conditions, in the absence of JA treatment the GmbHLH3-overexpression lines tended to have a reduced time to flowering. Moreover, GmbHLH3 transcript expression pattern analysis revealed that MeJA treatment could induce GmbHLH3 transcript in soybean. This study provides evidence that JA treatment alters the effects of GmbHLH3 on the regulation of plant growth and gene expression. It contributes to future research on the functions of bHLH TFs in soybean.