Photosensitivity plays an essential role in the response of plants to their changing environments throughout their life cycle. In soybean [Glycine max (L.) Merrill], several associations between photosensitivity and maturity loci are known, but only limited information at the molecular level is available. The FT3 locus is one of the quantitative trait loci (QTL) for flowering time that corresponds to the maturity locus E3. To identify the gene responsible for this QTL, a map-based cloning strategy was undertaken. One phytochrome A gene (GmPhyA3) was considered a strong candidate for the FT3 locus. Allelism tests and gene sequence comparisons showed that alleles of Misuzudaizu (FT3/FT3; JP28856) and Harosoy (E3/E3; PI548573) were identical. The GmPhyA3 alleles of Moshidou Gong 503 (ft3/ft3; JP27603) and L62-667 (e3/e3; PI547716) showed weak or complete loss of function, respectively. High red/far-red (R/FR) long-day conditions enhanced the effects of the E3/FT3 alleles in various genetic backgrounds. Moreover, a mutant line harboring the nonfunctional GmPhyA3 flowered earlier than the original Bay (E3/E3; PI553043) under similar conditions. These results suggest that the variation in phytochrome A may contribute to the complex systems of soybean flowering response and geographic adaptation.