Stigma exsertion is an important trait that contributes to the improvement of seed production in hybrid rice. We demonstrate that GS3, one of the genes regulating seed length, also regulates stigma length and participates in stigma exsertion in rice. GS3 mRNA is expressed in the basal part of the young stigma, and a nonsense mutation in the second exon of GS3 causes an increase in cell number, resulting in elongation of the stigma. Manipulation of GS3 should contribute to the improvement of hybrid seed-production efficiency.