Soybean (Glycine max) is a short-day crop whose flowering time is regulated by photoperiod. The long-juvenile trait extends its vegetative phase and increases yield under short-day conditions. Natural variation in J, the major locus controlling this trait, modulates flowering time. We report that the three J-family genes influence soybean flowering time, with the triple mutant Guangzhou Mammoth-2 flowering late under short days by inhibiting transcription of E1-family genes. J-family genes offer promising allelic combinations for breeding.