The phenomenon of multiple-grain spikelets is frequently observed in gramineous crops. In the case of dual-floret spikelets, the upper fertile floret develops normally to form a single grain, while the lower sterile floret undergoes abortion. Here we elucidate the role of Double-Grain 1 (DG1), a gene encoding a homeobox-domain-containing protein, in regulating the lower floret meristem activity and double-grain spikelet trait in sorghum. A 35.7-kb paracentric inversion in the DG1 promoter region leads to increased DG1 expression, probably by reducing repressive histone modifications. This increase in DG1 expression transforms the degenerated lower floret into a fertile one. The use of the superior DG1 allele results in an increase of approximately 40.7% to 46.1% in grain number per panicle and a 10.1% to 14.3% increase in overall grain yield. Our findings shed light on the sorghum double-grain spikelet characteristic, offering valuable insights for high-yield breeding designs in cereals.